Introduction
Circuit Description
K9GDT 40m CW/SSB Receiver
7-7.3 MHz
First Mixer
SBL-1
-6 db
Low Noise
Post Mixer
Amplifier
3.395 MHz
8 pole
Crystal Filters
500Hz
Filter
Gain EQ
I.F.
Amplifier
2.1 kHz
BPF SK3218 40673
MC1350 x2
400Hz
10.395-10.695 MHz
+7dbm
+15V
AGC
5.2v no signal
5.7v -10 dbm
LO Pre-mix
SBL-1
6-6.3 MHz S-meter
AGC
AC
4395 ±4 KHz
Passband Tuner
SBL-1
BFO Generator
3395 ±4 KHz
AC
1 MHz
L.O.
Display Mixer - 40673
Post I.F.
400Hz Filter
-6db
Pad
400Hz
Product
Detector
MC1496
BFO
3395 ±4 KHz
7258.0
BFO
7-7.3 MHz Frequency
Counter
7216C
Tuneable AF
Notch
400-3000 Hz
+5V
+12v
+15v
+5v
+22v
Switched
Capacitor
L. P. Filter
LPF
3.0 kHz
A.F.
Amplifier
3.5W 8
LM383
+22V
Power Supply by George!
7812
7815
7805
LM388AK
Version 3.1
1/94
Figure 2-1 Receiver Block Diagram
2-1 Bandpass Filter Module
The Bandpass Filter module has an input and output impedance of 50
Ω
and consists of two major sections:
a dual resonator parallel tuned bandpass filter and a seven element low pass filter for additional attenuation of image frequencies from 13.79 through
14.09MHz. The module’s insertion loss is under 1dB.
2-2 Mixer - Post Mixer Amplifier
The mixer stage is a doubly-balanced diode-ring device made by Mini-Circuits Laboratories, model
SBL-1. The 10.395-10.695MHz local oscillator drive level is +7dBm.
IMD performance and eliminating the requirement for a diplexer. Because a very low noise figure is normally not necessary around 7MHz, the pad’s 6 dB insertion loss is tolerable.
A 6 dB pad is placed between the mixer output and the post-mixer amplifier, which is a low noise, high dynamic range broadband bipolar amplifier with a
50
Ω
input impedance. This arrangement assures a proper non-reactive termination at the mixer’s IF port for all frequencies present, thereby preserving it’s
The post-mixer amplifier has a gain of 20 dB using an
NTE278 transistor, which has an f
T
of 1.2 GHz and a noise figure of about 4 dB. This device is heat sunk and biased to a standing current of 70 ma. The amplifier’s output is matched to the 2 k
Ω
input impedance of the crystal filter which follows.
Page 3
Circuit Description
2-3 Crystal Filter - IF Amplifier - AGC Module
Crystal Filters
The received signal is fed to this module and routed to one of two eight-pole crystal filters by steering diodes.
The bandwidths are selected by the front panel
SELECTIVITY control. For an IF bandwidth of
2.1KHz, this control places +15 VDC on the 2.1KHz
filter select input and GND on the 400Hz filter select input lines. To choose the 400Hz filter, the
SELECTIVITY control reverses this connection.
These filter selection signals are also supplied to the product detector for inserting a second 400Hz filter and to the IF amplifier for filter gain equalization.
when a strong signal appears while preventing most pops and noises from “hanging” the AGC system.
The AGC decay time is determined by the parallel combination of a timing capacitor and a 22
ΜΩ resistor at the JFET input of the AGC amplifier. The timing capacitor is one of three selected by the front panel AGC control. This arrangement provides AGC decay times of about 20ms, 500ms, and 5 seconds, thus allowing the operator to match the receiver’s
AGC characteristics to the received signal’s rate of fade. In addition, the front panel AGC control switch has an “OFF” position, which shorts the JFET’s input to ground, thereby defeating the AGC system.
IF Amplifier
The signal leaving the crystal filter is connected to a tuned amplifier (3.395MHz) which uses an RCA
40673 dual-gate MOSFET. The purpose of this amplifier is to establish the noise figure for the IF section and to provide gain equalization for the crystal filters. This low noise device provides ample gain that is easily controlled by a voltage at it’s second gate.
When the 400Hz filter is selected, +15 volts is applied to the GAIN EQ input, resulting in a 3.3 volt level on gate #2 for maximum gain. When the 2.1KHz filter is selected, the GAIN EQ input is grounded and the
MOSFET’s gate #2 voltage is then controlled by a
0-3.3 volt signal from the 2.1KHz FILTER GAIN EQ trim pot.
The AGC amplifier’s output is connected through a diode to the IF amplifier’s gain control line. Two other signals are similarly connected such that the most positive signal determines the receiver’s gain.
The front panel manual RF GAIN control raises the voltage on the control line to limit the maximum receiver gain with the AGC enabled. It will also provide total control of the receiver’s gain when the
AGC is defeated. The second signal mutes the receiver.
The signal is then amplified by a pair of cascaded
Motorola MC1350 amplifiers. These devices furnish most of the IF amplifier’s gain and over 120 dB of gain control. During no-signal conditions, the IF amplifier’s gain control line is set to 5.38 volts. Gain is reduced by increasing the gain control voltage.
Mute Control
The external mute control system is compatible with that used by the R. L. Drake Co. The receiver is muted when a companion transmitter removes the short to ground (normally supplied when not transmitting) from the EXT MUTE connector. This causes +12 volts to be placed on the IF amplifier’s gain control line, effectively silencing the receiver.
The EXT MUTE DEFEAT switch on the rear panel provides a ground to the EXT MUTE connector so the receiver can be operated without a transmitter.
AGC
The output of the IF amplifier is coupled to the AGC detector diode. This diode charges a timing capacitor through a resistor which establishes the AGC attack time of approximately 1 millisecond. This value is sufficiently fast to maintain a constant AF output
The monitor function also defeats the external mute feature by grounding the EXT MUTE input when the front panel function switch is turned to MON. This permits the operator to temporarily override the external mute for monitoring the transmitted signal or setting the transmit frequency.
Page 4
Circuit Description
2-4 400Hz Noise Filter/Product Detector
400Hz Noise Filter
A sample of the IF amplifier’s output is fed to the
400Hz noise filter/product detector for conversion to audio. When the SELECTIVITY control is set to
400Hz, the filter selection signal energizes two small relays that insert a 400Hz eight pole crystal filter before the product detector. This filter restricts the effective noise bandwidth to that of the first 400Hz filter. Without it, broadband noise generated by the IF amplifier will reach the product detector and produce an audio output.
When the SELECTIVITY control is set to the 2.1KHz
position, the 400Hz filter is removed and a 6dB pad is switched in the signal path. The purpose of this pad is to match the insertion loss of the 400Hz noise filter, thus insuring identical signal levels at the mixer input at both selectivity settings.
Product Detector
The received signal is then routed to the modulating signal input port of a Motorola MC1496-P balanced modulator used as a product detector. The BFO signal of 3395±4KHz is terminated in 50
Ω
, attenuated to
850 mV p-p, and applied to the balanced modulator’s carrier port.
The audio signal from the output port is connected to a low pass filter which removes any RF energy from the product detector’s AF output. The frequency response of this filter, as well as subsequent stages, is limited to 3000Hz. The result of this effort is clean sounding audio that is free from hiss and high pitched background garbage caused by filter “blow-by” from
very strong adjacent signals.
The audio is then routed to the audio processing board.
The requirement for a noise filter is much less stringent for a bandwidth of 2.1KHz, so one is not included. Almost all of the broadband IF noise at this selectivity setting is removed after detection by limiting the bandwidth of the audio section.
2-5 Audio Processing Board - AF Power Amplifier
Audio Processing Board
The signal is sent through an input level set potentiometer to a LF356 low noise op amp, which functions as a 3KHz low pass filter. The amplifier’s output is routed through a passive 200Hz highpass filter to another LF356 serving as a tunable notch filter. a unity gain buffer to supply the required low-Z drive to the filter input stage. The other is used to provide a
“stiff” source of V
CC
÷
2 bias. The clock frequency is adjusted to 300KHz, which sets the filter’s cutoff frequency to 3KHz.
The notch filter provides an adjustable response null from 400Hz to 3KHz by summing normal audio with the output of a phase inverting bandpass filter. The notch filter is disabled by removing the audio signal to the input of the bandpass filter.
The signal is routed through another 200Hz highpass filter network to the switched capacitor lowpass filter section.
Finally, the signal is routed to the board’s output connector through another highpass/lowpass network
.
AF Power Amplifier
The signal from the Audio Processing board is sent through the front panel AF GAIN control to the AF
Power Amplifier board. The amplifier uses a National
Semiconductor LM383, which is powered by a 20 volt regulated supply and is capable of delivering 3.5 watts into an 8
Ω
load.
The switched capacitor filter uses a National
Semiconductor MF6-100 6th order Butterworth
Lowpass filter. This chip also contains two op amps and an on-board clock. One of the op amps is used as
The SIDETONE input connector on the rear panel allows the receiver’s power amplifier and speaker to be used by an accessory CW electronic keyer.
Page 5
Circuit Description
2-6 VFO
The VFO functions as the main tuning element by supplying a 6-6.3Mhz signal to the LO - Passband
Generator module. The signal is produced by a temperature compensated series-tuned Colpitts JFET oscillator with 6.3 volt zener diode regulation. Tuning is accomplished by a high quality double ball bearing variable capacitor driven by a zero-backlash flywheel weighted 50:1 gear reduction drive.
The oscillator stage is followed by a two transistor buffer amplifier that produces a 5 volt p-p signal.
2-7 LO - Passband Generator Module
Passband Tuner
The Passband Tuner is an oscillator similar to the
VFO described above. However, the 4395KHz output frequency can be shifted ±6KHz by a tuning voltage from the front panel PASSBAND control. This DC voltage is bias for varactor diodes which are part of the oscillator’s series tuned circuit.
There is a buffered Passband Tuner signal available at the PBT connector for use by the BFO Generator module.
The effect of the ±6KHz offset is to shift the BFO and
LO frequencies. The shifted LO repositions both the received signal and nearby interference around the response curve of the crystal IF filter. This permits the interfering signal to be placed down on the filter’s skirt where it’s effect will be less severe. Because the
BFO frequency is also shifted by the same amount and in the same direction, there is no change in the received frequency.
LO Generator
The 4395KHz output of the Passband Tuner is mixed with the 6-6.3MHz VFO output to produce the local oscillator signal of 10.395-10.695MHz. A Mini-
Circuits SBL-1 is used as the mixer and is followed a two transistor amplifier section with suitable bandpass filtering. The 50
Ω
, +7 dBm LO output is very clean, with mixed products and harmonic energy greater than 65dB below the LO carrier level.
A second, less spectrally pure, LO signal is available at the DISP connector for use by the Display Module.
2-8 BFO Generator
The BFO Generator furnishes a 3395±4KHz signal at
+7dBm to the Product Detector, which converts the IF signal to audio. The BFO signal is produced by mixing the 4395±4KHz PBT signal with the output of a 1MHz crystal-controlled offset oscillator. The difference frequency of 3395±4KHz is amplified by a two stage tuned amplifier.
A second BFO output is used by the Display Module for frequency readout.
2-9 Display Module
The display module contains circuitry for the signal strength meter and digital readout. The S-meter is connected to a conventional balanced bridge circuit with one side driven by the AGC amplifier. The traditional ZERO and FULL SCALE adjustments are present for meter calibration. will indicate approximately S3, with a 50µV input indicating S9. A 30mV input will bring the S-meter to full scale and signals of 100mV and greater will peg the meter.
When the receiver is properly adjusted, the S-meter will indicate S1 when the AGC is off and the RF
GAIN control is at it’s clockwise end of rotation.
When the AGC is enabled, noise output from the IF amplifier will “tickle” the AGC and cause the meter reading to nudge upward slightly. An input of 1µV
The digital frequency readout is a straightforward frequency counter which counts a 7-7.3MHz signal that is produced by mixing the 10.395-10.695MHz
LO and 3395KHz BFO signals. The counter is based on an Intersil 7216C counter chip. Extensive shielding and decoupling are used in this and other modules to minimize leakage from the Display
Module to the receiver front-end circuitry.
Page 6
Circuit Description
2-10 Power Supply
The power supply consists of four regulated outputs of
+5, +12, +15, and +20 volts. The +5 volt output is used to power the TTL and CMOS chips used in the frequency display circuitry. The +15 volt section is used to supply power for the crystal filter selection diodes, the local oscillator output amplifier section, and the post-mixer amplifier. The AF power amplifier is operated from the +20 volt section, which is a National
Semiconductor LM338AK 4 amp voltage regulator.
The remaining circuitry is operated from the +12V section. LM78XX series 1 amp regulators are used for the +12, +15, and +5 volt outputs.
Digital Frequency Readout
Signal Strength Meter
Main Tuning
Notch Tuning
Passband Tuning
AC Power - AF Gain Control
AGC Control - Off/F/M/S
RF Gain Control
Figure 2-2 Front Panel Layout
Selectivity Control
2.1 kHz - 400Hz
Mode Switch
Monitor - Normal - Notch Enable
8 Ohm Speaker Outputs
1/4" Headphone jack or
RCA Connector.
Antenna Input
External Mute Defeat Switch
External Mute Input
Sidetone Input
Figure 2-3 Rear Panel Connections and Controls
AC Line Cord
Fuse 3/8 amp Slo-Blo
Page 7
AF Power Amplifier
Power Supply
Display Module
Circuit Description
Figure 2-4 Interior Views - Module Locations
VFO
Bandpass Filter
Crystal Filters
AF Processor
Board
IF Amplifier/ AGC
Mixer - Post Mixer
Amplifier Module
Top View
LO-Passband Generator Module
Audio Processing Board
BFO Generator
VFO
Bandpass Filter
Power Supply
Bottom View
400 Hz Noise Filter/ Product Detector
Page 8