Racal Sideband Adapters … a comparison

February 2017: I thought it would be appropriate to take a comparative look at all four Sideband adapters. All four are made up of the same common building blocks. So what distinguishes one from the other? They all have two Local Oscillators making them effectively dual conversion receivers operating at a fixed input frequency of 100KHz (with one or two exceptions … see below)

Mixer

Local Oscillator

82KHz or 118KHz

Oscillator

18KHz

Product

Detector

Band-pass

Filter

15 - 21KHz

High-pass

Filter

18.5KHz

Low-pass

Filter

17.5KHz

Low-pass

Filter

3.5KHz

Audio

Output

I.F.

Input

RA63C (465KHz IF) Local Osc. 447KHz

RA63E&L (455KHz IF) Local Osc. 437KHz

With only 6 valves, and one of these is a rectifier, the RA63 is the simplest of the four Sideband adapters. It is also the only one that can truly be called an SSB adapter since all the others are essentially Independent Sideband Adapters. The RA63 really was designed to satisfy the need for SSB reception and little else.

There are no crystal oscillators involved (with the exception of the D & K variants). Both Oscillators are relatively simple, yet stable L-C oscillators with the 1st LO 18KHz below the 100KHz IF input at 82KHz. Fine tuning is achieved over a range of +/- 1KHz by tuning the 1st LO.


There are however variations. The RA63C was designed for an IF of 465KHz whilst the RA63E catered for an IF of 455KHz. The RA63D and K both employ crystal controlled oscillators with the 1st LO oscillator being temperature controlled and running at 118KHz, and the 2nd LO using the same dual-element 18KHz crystal as in the RA98 and RA218. Here, the LP and HP filters are accordingly swapped since the 1st LO is now above the 100KHz IF.


September 2017: Whilst repairing an RA63H and and RA63K I was able to perform some tests around the filters. Unlike most modern SSB receivers, Racal’s adapters do NOT employ crystal lattice filters. Instead, the Sideband filters use tuned LC elements. This is probably why an IF of 18KHz was chosen. This also eliminates the often unwanted side effect of ‘ringing’. It is probably overlooked by many, that the Low-Pass and High-Pass filters themselves only satisfy part of the converter’s bandwidth specification. In essence the actual bandwidth of the Sideband Adapter is governed not by these complex filters but by the much simpler Band-Pass filter that precedes them. The Sideband filters provide an effective ‘brick wall’ at the LF end of the response with the Bandpass filter providing the upper limit of the response. In the RA63 and RA218, the BPF response is given as 15KHz - 21Khz; thus, the effective bandwidth is 3KHz (nominal). See the diagram on the next page for an explanation.

RA63 Block Diagram