Racal SA.97 Wobbulator page 6

Finally … The page you’ve all been waiting for. On the left is a photograph of the trace of a real 40MHz filter which was previously aligned using my DG8SAQ VNWA. Admittedly the pass-band ripple looks a bit excessive. However, much of that ripple can be attributed to cable length and termination. The markers are also not yet set correctly but the object of the exercise was to get a meaningful trace on the CRT and I think you will agree, that’s what we have here! … I’m dead chuffed.


As previously mentioned, the RF output from the Sweep Generator and the RF input to the Lin/Log Amp are unusual in that although they resemble the small connectors found on the rear of RA17s, they are in fact different since the centre conductor is a receptacle as opposed to a pin.


As can be seen from the photograph below, I managed to obtain a much better trace by fashioning short connections between the connector outer and the coax braid. In time I shall look into making dedicated leads for connection to TP2 and TP3 on the RA17. But until then I think you will agree it is quite fascinating to see this bespoke piece of test gear actually  working.  

More on the Marker Generator, left:

I now have the 5 markers correctly set. This was actually very easy to do. The way it works is remarkably simple. The smaller of the two variable inductors is used to set the centre marker on 40MHz (the Crystal Oscillator). The coax lead carries the auxiliary swept signal from the sweep oscillator. The taller of the two cans contains two tuned circuits in series which are themselves in series with the anode of one of the EF91s. I haven’t figured out precisely how it works, other than I can tell that some form of mixing is taking place, but the bottom core adjusts the +/-650KHz markers while the top core sets the +/- 1.5MHz markers. The markers were set by feeding known signals at 40MHz, 40.65MHz and 41.5MHz into the Lin/Log Amp input and adjusting the three cores. Signals at 39.35MHz and 38.5MHz were then applied to verify the other two markers.