MFJ MFJ-784 User Manual

	MFJ-784 Instruction Manual
Table of Contents
Filter Specifications ...................................................................................................................................	1
Notch Filters:.................................................................................................................................	2
Random Noise Reduction Filter:...........................................................................................	2
Signal Processor: ........................................................................................................................	2
General Features: .......................................................................................................................	2
Rear Panel: ....................................................................................................................................	2
Introduction to DSP...................................................................................................................................	3
Connections..................................................................................................................................................	4
Input Connections.......................................................................................................................	4
Output Connections ...................................................................................................................	5
Connecting to Receive Audio .................................................................................................	6
Power ...............................................................................................................................................	6
Headphones Out..........................................................................................................................	6
Speaker Out...................................................................................................................................	6
Filtered Audio Out .......................................................................................................................	6
Input Level Adjust ........................................................................................................................	7
Receive Audio In...........................................................................................................................	7
To Radio and To TNC.................................................................................................................	7
Front Panel ...................................................................................................................................................	8
AGC On-Off......................................................................................................................................	8
Program..........................................................................................................................................	8
Custom Tunable/Pre-Set Filters..........................................................................................	8
Filters................................................................................................................................................	9
Tunable Filters..............................................................................................................................	9
Notch ON - OFF ............................................................................................................................	9
Notch AUTO - MANUAL............................................................................................................	9
Noise Reduction ..........................................................................................................................	10
Noise Reduction On - Off ..........................................................................................................	10
Volume.............................................................................................................................................	10
Power On - Off...............................................................................................................................	10
FILTER IN - FILTER OUT.............................................................................................................	11
Filter Description........................................................................................................................................	12
LR/HR [1] ......................................................................................................................................	12
LR/HR [1] (Bandstop) ..............................................................................................................	12
BP [2] ...............................................................................................................................................	13
2BP [3] ............................................................................................................................................	14
CW [4] .............................................................................................................................................	14
SSB [5].............................................................................................................................................	15
RTTY [6] ..........................................................................................................................................	16
HF PACKET [7] .............................................................................................................................	16
AMTOR [8] .....................................................................................................................................	16
PACTOR [9]....................................................................................................................................	16
SSTV/ FAX/ WeFAX [10]......................................................................................................	17
NOTCH.............................................................................................................................................	17

	MFJ-784 Instruction Manual
DSP Operation.............................................................................................................................................	18
Initial Operation ............................................................................................................................	18
CW Operation...............................................................................................................................	19
SSB Operation ..............................................................................................................................	20
Custom Filters.............................................................................................................................................	21
Programming a Custom Filter ..............................................................................................	22
Jumper Settings.........................................................................................................................................	24
CW Sidetone Filter Settings...................................................................................................	24
DATA Default ................................................................................................................................	24
In Case of Difficulty ....................................................................................................................................	26
Technical Assistance................................................................................................................................	26

MFJ-784 Instruction Manual

MFJ-784 tunable DSP Filter

The Filter Specifications chart lists the parameters for the ten filters available on the MFJ-784. Definitions for the abbreviations used on the chart are listed below:

LR:	Low Reject Cutoff Frequency	fc:	Center Frequency
HR:	High Reject Cutoff Frequency	BW:	Bandwidth
f1:	Center or Notch Frequency #1	FIR:	Finite Impulse Response
f2:	Center or Notch Frequency #2	IIR:	Infinite Impulse Response

Filter Specifications

Tunable Filter	Left Control	Right Control		Attenuation	Type
LRHR1	LR: 200-2200 Hz	HR:1400-3400 Hz		60 dB @ 74 Hz	FIR linear phase
				outside passband
BP	fc: 300-3400 Hz	BW:	30-2100 Hz	50 dB @ 60 Hz	FIR linear phase
				outside passband
2BP2	f1: 300-3400 Hz	f2:	300-3400 Hz	50 dB @ 60 Hz	FIR linear phase
				outside passband
CW3	fc: 300-1000 Hz	BW:	30-700 Hz	50 dB @ 60 Hz	FIR linear phase
				outside passband
SSB4	fc: 600-1700 Hz	BW:1000-2500 Hz		60 dB @ 74 Hz	FIR linear phase
				outside passband
Manual Notch	f1: 150-3400 Hz	f2:	150-3400 Hz	40 dB @ 95 Hz	IIR
				outside passband

All FIR filters have a 24 mS delay time and have the highest cutoff frequency limited to 3900 Hz.

Notes

1The LR HR filter becomes a band-stop filter when LR is adjusted higher than HR.

2The 2BP filter uses the bandwidth setting last used in BP filter but allows independent variation of the two center frequencies.

3The CW filter has an optional jumper-programmable sidetone filter, see page 22

4The SSB filter has its lowest cutoff frequency limited to 175 Hz.

Pre-Set Filter	Mark-Space Freq.	Bandwidth		Attenuation				Type
RTTY	Jumper Programmable	250 Hz	50	dB	@	60	Hz	FIR	linear
			outside passband					phase
HF PACKET	Jumper Programmable	540 Hz	50	dB	@	60	Hz	FIR	linear
			outside passband					phase
AMTOR	Jumper Programmable	340 Hz	50	dB	@	60	Hz	FIR	linear
			outside passband					phase
PACTOR	Jumper Programmable	440 Hz	50	dB	@	60	Hz	FIR	linear
			outside passband					phase
SSTV/FAX/WeFAX	Dual filter with passbands	300 and 900 Hz	44	dB	@	60	Hz	FIR	linear
	fixed @ 1050-1350 Hz and		outside passband					phase
	1450-2350 Hz

*All pre-set filters have a 24 mS delay time.

*The data filters allow 170 or 200 Hz shift mark-space frequency; see page 24.

MFJ-784 Instruction Manual

Interference Filter	Frequency Range	Attenuation	Type	Delay
Multiple Automatic	Entire freq. range of the	Up to 50 dB, varies with	Adaptive	8 mS max.
Notch	received audio	stability of the heterodyne
Random Noise	Entire freq. range of	Up to 20 dB, varies with the	Adaptive &	8 mS max.
Reduction	selected bandpass filter	characteristics of the noise	adjustable

Notch Filters:

Multiple automatic notch attenuates up to 4 tones.

Manual notch attenuates 2 separate tones with the front panel knobs. Automatic Notch is disabled in the following modes:

CW, RTTY, HF PACKET, AMTOR, PACTOR, SSTV/ FAX/ WeFAX

Random Noise Reduction Filter:

Adaptive and manually adjustable algorithms provide up to 20 dB noise reduction.

Signal Processor:

Signal Processor:	Analog Devices ADSP-2105
Clock:	12 MHz
Data Width:	16 bits

General Features:

Direct audio bypass when power switch is in "off" position. Audio output power is 1.2 w into 6 ohms (MFJ power supply). Audio frequency response is 250 to 3500 Hz (-3 dB).

Maximum audio input at full sensitivity is 3 volts P-P nominal (190 mW 6 ohms). Input circuit loading is 10 K ohms nominal.

Rear Panel:

Power:	10-16 Vdc @ .35 amp peak (low "Z " audio load)
Headphones Out:	1/4" stereo (or mono) phone jack
Speaker Out:	3.5 mm mono phone jack
Receive Audio In:	RCA phono jack
Input Level Adjust:	screwdriver adjustable potentiometer
Filtered Audio Out:	RCA phono jack (~1.5 V P-P @ 600 ohms)
To Radio:	5 pin DIN female (filtered audio out, PTT sense, receive audio in).
To TNC:	5-pin DIN female (filtered audio out, PTT sense, receive audio in).

MFJ-784 Instruction Manual

Introduction to DSP

The MFJ-784 tunable DSP Filter uses state-of-the-art Digital Signal Processing (DSP) technology. Digital Signal Processing greatly improves signal clarity by reducing or eliminating noise (QRN) and interference (QRM). DSP technology has existed for many years but has always been very complicated and expensive. Recent advances in integrated circuits have greatly increased the processing power and reduced the size of DSP units. These same advances also lowered the cost of DSP filtering, making DSP technology affordable for the average amateur or short wave listener.

Almost any microprocessor can perform DSP, but only very fast or special-function processors perform DSP in real time. Therefore the heart of any DSP system is the digital signal processor. A digital signal processor is similar to the CPU in a conventional home computer, but its commands are tailored to the type of instructions used in signal processing. The use of special DSP commands allows a DSP filter function to be completed in very few clock cycles (usually one); where a typical home computer CPU would require a long set of instructions and therefore many clock cycles to perform the same function. Analog Device's ADSP-2105 16-bit processor is used in the MFJ-784.

The MFJ-784 DSP Filter converts the analog input signals from your receiver to digital information. This conversion is achieved through the sampling of the signal many thousands of times per second with an A-to-D converter. The result is a string of digital "numbers" that represent the amplitude and frequency of the analog input signal. The ADSP-2105 chip then processes the resulting digital information with different digital filter algorithms depending on which one the user selects with the front panel controls. The end result is a digitized signal with undesired signal components either reduced or removed. The processed digital signal information is converted back to an audio signal by a digital-to-analog converter and sent to the audio outputs and speaker.

MFJ-784 Instruction Manual

Connections

The illustration below shows the DSP's rear panel connectors.

Power: The power connector connects the MFJ-784 to a 12 - 16 Vdc, 350 mA (or larger) power supply.

Headphones Out: This jack connects the MFJ-784 to your station headphones or speaker. Inserting a plug in this jack disables the SPEAKER OUT jack.

Speaker Out: This jack connects the MFJ-784 to your station speaker. A connection made to the HEADPHONES OUT jack disables this jack.

Filtered Audio Out: This jack connects the MFJ-784 to the external audio input jack of a radio or other accessory. This jack has a fixed level output independent from the DSP's volume control.

Input Level Adjust: This adjustment varies the sensitivity of all audio inputs. Receive Audio In: This jack connects the MFJ-784 to the audio output of the radio.

To Radio: This jack supplies connections to the filtered audio output and the receive audio inputs. In addition, a connection is available for a PTT sense line to automatically bypass the filter during transmit.

To TNC: This jack supplies connections to the filtered audio output and the receive audio inputs. In addition, a connection is available for a PTT sense line to automatically bypass the filter during transmit.

Input Connections

In most cases, the MFJ-784 will be installed between the speaker or headphone audio output of the receiver and your headphones or speaker. This diagram shows the typical connections used to attach a radio to the MFJ-784. Only one of the method of connection may be used.

MFJ-784 Instruction Manual

Output Connections

MFJ-784 Instruction Manual

Connecting to Receive Audio

The MFJ-784 Receive Audio In jack connects directly to your receiver's headphones or speaker jack. A minimum of 2 volts P-P (peak-to-peak) is required for full audio output when the Input Level Adjust is set for maximum sensitivity.

Audio input peaks that exceed 4 volts P-P will cause distortion, especially with the AGC or noise-reduction enabled. This distortion may occur while the audio output control of the receiver is at a normal gain setting. This distortion may be minimized or eliminated by adjusting the Input Level Adjust. See the CW and SSB operation sections starting on page 19 to adjust the input level or receiver volume.

Power

The Power connector is located on the left rear panel of the MFJ-784. This connector accepts the dc input that powers the MFJ-784. The Power connector of the MFJ-784 accepts a 2.1 mm coaxial plug with a positive polarity center conductor. An optional dc supply, the MFJ-1315, is available from MFJ.

The MFJ-784 requires a filtered voltage of 12 to 16 Vdc with a negative ground (or ground independent) power source. The maximum current demand changes with the volume level and the impedance of the load. The maximum current demand will be less than 350 mA at maximum volume but will always be more than 175 mA. If supply voltage drops below 11 volts the MFJ-784 will enter a power down reset mode and go "dead".

WARNING: Application of reverse polarity or voltages greater than 18 volts may permanently damage the MFJ-784.

Headphones Out

The Headphones Out jack can be used to connect the MFJ-784 audio output to headphones or a speaker. This jack accepts a standard male 1/4 inch stereo or mono headphone plug. The tip supplies the monaural audio and the ring supplies the ground. This allows the use of either stereo or monaural headphones, or a speaker. The Speaker Out is disabled when a plug is inserted in the Headphones Out jack.

Speaker Out

The Speaker Out jack accepts a 3.5 mm phone plug. This jack provides the same audio output as the Headphones Out jack. The "hot" audio signal appears on the tip. This jack will function with either mono or stereo plugs, but only the tip will supply audio. This jack is automatically disabled whenever a plug is inserted into the Headphones Out jack.

Filtered Audio Out

The Filtered Audio Out connector is a standard RCA phono jack. A quality shielded cable should be used for connections to the Filtered Audio Out jack. This jack provides approximately 1.5 volts P-P into 600 ohm (or higher) impedance loads. The output voltage of this jack is not affected by DSP volume control knob adjustment. The voltage level at the Filtered Audio Out jack is dependent on the receiver's audio output level when the DSP's AGC is turned "off" or if the DSP is bypassed. When the AGC is operating the voltage will remain nearly constant at 1.5 volts P-P.

MFJ-784 Instruction Manual

Input Level Adjust

The Input Level Adjust controls the sensitivity of the MFJ-784 receive audio and the audio input pins (pin 4) of the DIN connectors. Proper adjustment is achieved if the receiver volume control can be set at the normal setting without overdriving the DSP (with the AGC and/or the noise reduction operating). The DSP's volume should barely change as the AGC is turned "on" or "off". The chart below describes symptoms that will appear if the input level is adjusted to an incorrect level. See the CW and SSB operation sections starting on page 19 to adjust the input level or receiver volume.

Symptom	Line Level Adjustment
turn AGC "off", volume decreases slightly	No Adjustment
turn AGC "off", volume decreases significantly	Increase
turn AGC "on", volume increases significantly	Increase
audio distortion	Decrease
cracking or popping on signal peaks	Decrease

Receive Audio In

The Receive Audio In connector is a standard RCA phono jack. A shielded cable should be used to connect the Receive Audio In connector to the station receiver. This jack should be driven with a minimum of 2 volts P-P, and no more than 3.5 volts P- P unless the setting of the Input Level Adjust is reduced (counterclockwise adjustment from rear view). This jack has an impedance of approximately 10,000 ohms, and is normally connected to the receiver's speaker or headphones output.

To Radio and To TNC

The To Radio and To TNC ports are standard ports used by MFJ products. These ports allow the DSP unit to be connected to MFJ or other brands of TNC without disturbing the speaker or headphone connections. These ports can be connected to virtually any radio with one of the pre-assembled MFJ-50xx series cables (available from MFJ). Each cable in this series is wired for a specific brand of radio.

The To Radio and To TNC ports also provide connections to the PTT line input. All signal processing functions are bypassed when this line is pulled low. Pulling the PTT line low allows the transceiver's sidetone and audio monitoring functions to appear at the DSP output without modification. The PTT line has a blocking diode to prevent other equipment connected to the PTT line (such as a linear amplifier) from damaging the DSP. Refer to the CW section starting on page 14 for a description of PTT configuration.

WARNING: Never connect the PTT line to negative voltages or to positive voltage sources that exceed 35 volts. If a linear amplifier is connected to the PTT line, a blocking diode (1N4001 or equivalent) should be connected from the linear amplifier's control (relay) jack to the PTT line. This diode prevents the amplifier from loading the DSP's PTT line when the amplifier is turned "off". The anode of the diode should be connected to the amplifier and the cathode (banded end) to the PTT line.

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