The LM373 ~ A 20 Meter SSB Transceiver -- 1970 Revisited


The LM373 SSB Transceiver

This Odyssey started in the early 1970's with a QST article authored by Tom Sowden, now W6KAN, that featured a single IC SSB receiver, the LM373 produced by National Semiconductor. This IC was a complete AM/FM/SSB IF strip that came in both a 10 Pin TO-5 style can and a 14 Pin DIP . Tom's receiver covered two bands (80 & 20 Meters) with a 9.0 MHz IF and had a 40673 DGM Mixer Stage and followed by an IC Audio Amp. When I saw that article I was hooked, especially since my brother-in-law worked at National and sent me several LM373 samples to "play with". As luck would have it my next door neighbor was a ham --Bill, K6ACJ. Together we decided to build a SSB transceiver based on using W6KAN's circuit for the basis as a transceiver. We were successful and we both had transceivers operating on 20 Meters.

In 1973 Charley Hill, W5BAA published an article in the November Ham Radio magazine which covered the use of the LM373 in a 20 Meter SSB transceiver. W5BAA used three LM373's in his design with one as the main IF stage and the second as the receiver mixer and the last LM373 as the balanced modulator. His design had many refinements over what I used. I even contacted Charley and he was kind enough to send me some additional notes where he had made some further refinements.

Fast forward to 2017 and I had the desire to once again create that 1973 transceiver based largely on Charley's design and we now have an operating radio. I will attempt to document what I did so that maybe others will have a fancy to create (or re-create) a 1970's SSB transceiver. This was the very first solid state SSB transceiver that I built! I reconnected with W5BAA and he sent me the original article plus some data sheets that cover the LM373.

My plan is to present the main circuit element in W5BAA's article and then document what I changed. So lets start with the basic IF strip. Below is a page from the article and the only thing I kept was the diode steering and the main LM373 IF element. For the Receiver Mixer we used the SBL-1 and for the Balanced Modulator we used a second SBL-1. The LO and BFO were supplied by the Si5351. So these changes will greatly simplify the build. For the front end filtering I used a Band Pass Filter I designed using LT Spice. A second Band Pass Filter (same elements as the 1st one) is used on the transmit output. A few extra parts but no switching required.

Long ago I came up with a design for a bi-directional stage having two separate 2N3904 amplifier stages. One half is employed as the Receiver RF Amplifier stage and the other half is the Transmit Pre-Driver. The stage gain for each leg is manually set. The Transmit Driver stage is straight out of EMRFD but uses a 2N2222 (TO-18 Style) and a BD139. All other circuit elements are as in the 2N3904/2N3866 EMRFD Design. This stage can be run at about 400 Milli-Watts.

The audio amp is a 2N3904 driving an LM386 and the microphone amp is a single 2N3904. Both of these circuits are built on a common PC board and located near the front panel so that lead lengths and control wiring is short. This rig has a microphone gain pot on the front panel.

Also noteworthy is the main power source to the rig is via a regulated 11 VDC and the only two boards operating from 13.8 VDC are the Final RF Amplifier which is the IRF510 and the Si5351 PLL Clock Generator. I should mention the Si5351 has a 9 VDC regulator built into the board assembly. Whle I could run the Si5351 off of the 11 VDC supply I wanted to lessen the load on the TIP32C transistors that switch the 11 VDC supply to the Receive Transmit circuits. The second special board was the N6QW designed solid state switch. For tune up I generate a 988 HZ tone in the Arduino and feed that into the balanced modulator after passing through a three stage RC filter so it is more of a sine wave versus a square wave.

A short tutorial here in the form of a magic decoder ring to map the pin connections for the TO-5 Style and the 14 Pin Dip. Yes the info is in the data sheets but as often happens, the anxious builder simply presses on and then emails me his circuit doesn't work. So READ THIS straight away. The 14 Pin DIP version has been frequently been seen on eBay.


Pin TO-5 14 Pin Dip
2 1st Input 1st Input
3 DC Feedback DC Feedback
4 2nd Input 2nd Input
5 Ground
7 Product Detector Ground
8 Peak Det Output SSB BFO
9 1st Section Output Product Detector
10 Vcc (11 VDC) Peak Det Output
12 1st Section Output
14 Vcc (11 VDC)


= No Connection or Not Connected!


In the following sections I will provide schematics and detailed photos of the boards and an overall block diagram of my approach to the 2017 LM373 20M SSB Transceiver.

A *
Band Pass Filters and Front End SBL-1
Balanced Modualtor SBL-1
Bi-Directional Amp Stage
IRF510 Final Amp
11 VDC Regulator and Solid State Switch