1. “Is it time to ban the 458?” or: Transceive Operation with the Central Electronics 20A
By Robert Nickels W9RAN
© 2017 RAN Technology Inc.

2. Single-conversion SSB exciter: Develop SSB signal at 9 Mhz
Use external VFO or crystal to mix 9 Mhz SSB to desired frequency
Central Electronics 20A
Speech
Amp
90° AF Phase Shift Network
Diode
Modulator
9 Mhz
Osc.
90° RF Phase Shift Network
Mixer
Driver PA
Xtal or VFO

3. “-” indicates SB reversal
CE 20A Mixing Scheme
“-” indicates SB reversal
Injection to the mixer can be either above (high side) or below (low side)
Sidebands will be reversed (SB1 vs SB2)
Mixing products and spurs will be different for each
Norgaard “5 +/- 9” scheme is easiest and most efficiently produced with converted command set transmitter as VFO, reduced output on other bands
Short version: Operating Freq. plus or minus 9 Mhz into the 20A

4. Double-conversion superhet receiver:
VFO and heterodyne crystals are “premixed” to produce injection signal that is Mhz above the desired signal.
Crystal filter at Mhz
2nd conversion and LC filter at 50 Khz.
Drake R4 series

5. Desirable Characteristics
Excellent SSB receiver
Injection output (premix system)
Drake premix internally mixes the VFO with a crystal oscillator to produce a first LO injection signal that is always Mhz above the desired operating frequency
Simplifies operation with a transmitter in transceive mode
Drake’s choice of INJ frequencies is fairly compatible with the 20A requirements

6. Drake R4 series analysis
Nominal LO of Mhz works for 80 and 20 meters and is reasonably clear from image frequency
= 5250 (SB1)
= (SB2)
Others may require additional filtering or amplification or both
Spurs and spectral purity TBD (look at how many traps Wes had to use)

7. Sources of Frequency error, drift, etc
All crystals age differently and unpredictably
Different errors to begin with
Not high precision
Temperature effects depend on operating conditions

8. Solution: Frequency agility
NE-602 Mixer
Class A Amp
R4 INJ OUTPUT
20A VFO Input
~20V P-P
Si5351
Clk Gen
Up/Down
switches
Arduino Nano
12VDC

9. Results (so far) GOOD LESS GOOD
Provides the essential ability to set frequency and compensate for drift
Simple, easy to build, low-cost (~<$50)
Freq. increment can be set as desired (20Hz steps presently)
Non volatile memory holds last-programmed value
Drives 20A to full output on 80 and 20 meters
LESS GOOD
Range of INJ and required LO frequencies make covering some bands challenging with the R4 series
The same is true for most other receivers
20A has reduced efficiency on bands other than 80 and 20
Solving the above issues is probably possible but spectral purity is unknown

10. Conclusion
A modern Arduino-controlled digital frequency generator provides excellent transceive operation with the CE 20A and Drake R4 series receiver on 80/20 meters, and is easily reproducible.
Let’s get more 20As on the air!
Remember – if Wes hadn’t been willing to incorporate new technology into his products it would have been: AM