Improving Selectivity In Vintage Receivers

[Jon_G4MDC]

I think that might be a fair way to start. The emitter and collector positions might be good places to try adding single ceramic resonators if that's what you have.

If you have 3 or 4 terminal filters then they would probably be better in series with (or in the place of) the IF gain control. I suspect you will need a terminating resistor though and watch to block any DC in both these scenarios!

Sorry still no results for Murata SSB filters. You know who's law says I have lost it...grr.

[Bazz4CQJ]

Well, that IF board has been doing nothing for a very long time (might not work!), and although the wobbulator may not be rocket science, I guess there will be a learning curve to discover the optimum setup, so the board can be used for that. Then, I'll start adding the filters. While ever the ambient temperatures stay close to zero, playing "ham radio" seems like reasonable choice of activity .

Losing things is very frustrating and I do it all the time.

B

[GW3OQK Andrew]

Hello,
More on my HRO. I removed the filter I had fitted between 1st and 2nd IF, but 1st IF amp still remained biased. Just to be clear, there was no wire connection from the IFT going to the 1st IF amp grid. A 1 microvolt signal at 3.6MHz could still be heard clearly from the loudspeaker. With that amount of feed-through its no wonder the skirt bandwidth is so wide. The crystal filter works well but on CW you can hear signals out to +/- 5Kcs and I suppose that's what it was always like. You ears select the note you want.

I once had a Drake R4C receiver. You could unplug the crystal filter and still receive signals, though I improved that somewhat by fitting adhesive aluminium tape to various wires and ground as screening.

73, Andrew

[Jon_G4MDC]

That is definitely something to ponder. Until that is stopped putting a selective filter there would be pointless.

[Bazz4CQJ]

Yes, Andrew's observation is quite an eye opener, but does fit in with some previous experience. In the days when I used my HRO behind a 2m converter, and tuning 4-6MHz, I was shocked to find it really did interfere with the family (Band 3) TV. Lots of RF came out of the HRO and went straight down the mains and at sufficient strength to affect the TV. I seem to recall there were no decoupling caps on the LT wiring. Adding a few 'changed the picture completely' .

Furthermore, it's certainly the case that the layout of the HRO was not optimised around the position of the RF filter and the mixer and first IF valves.

B

[suebutcher]

Originally Posted by joebog1

Bazz, there were also some available that took the place of a traditional transistor IF transformer.
They have five pins and used to be coloured red. They are something less than 1/2" high and wide, and about 1/4" thick.

Joe

Murata SFD-455B. I have too many of these, so if anyone would like some samples let me know.

[Bazz4CQJ]

Kind offer Sue, but are those particular ones 10kHz wide at -3dB, in which case I think they are for domestic radios?

B

[suebutcher]

Looks like it. You've have to use several, I guess. The "top coupling" capacitor value adjusts the bandwidth, but I'm not certain how or by how much.

[Radio Wrangler]

The pattern of the values of top coupling capacitors between resonators sets the filter shape, Butterworth, Chebyshev, Gaussian, etc.

The scaling of the coupling capacitors (with respect to the resonator capacitors) in cahoots with the load and source impedances coupled into the ends of the filter - to set the Q of the first and last stage all together set the bandwidth (with respect to the centre frequency)

The tuning of the resonators sets the centre frequency.

These 'coupled resonator' filters start out with all the resonators identical and tuned to the same frequency (geometric mean of the 3dB points of the finished filter) but the piulling effects of the coupling capacitors hve to be taken into account (consider all resonators but the one you are looking at to be shorted) and trim the one you're looking at to the planned centre frequency. Do each one in turn with the others shorted. And you now have the filter resonators adjusted. With the shorts off, each resonator gets pulled off of its centre frequency, and its Q loaded to give exactly the pole-zero pattern for the wanted filter.

The nasty bit is if you start sweeping the filter, you will see as many bumps as the filter has resonators. Aha! you think, each resonator trimmer must control one bump!

No such luck, each resonator controls a pair of bumps and interacts with all the others. The shorting trick is the quick way. Try just twiddling, even with a swept display live in front of you and you'll tend to diverge from the wanted alignment and it just keeps getting worse.

Inside the ceramic filter, someone designed the coupling factors and thence the capacitors. They then offset the lengths the resonators were ground to, to correct the tuning. This is the one setting you can't fiddle with downstream. The bandwidth is designed into the filter. If you vary the source and load impedance, you'll worsen the ripple. If you vary the coupling capacitors, you can vary the bandwidth but you can't fiddle with the resonator trimming to counteract the ripple getting worse.

So, for a filter with a controlled shape, you're stuck wiith the designed-in bandwidth, and you need to be fairly close with the source and load impedance if you want the ripple in-spec.

These hints go for all coupled resonator filters. LC, Quartz, Ceramic, waveguide, helical. as well as all the alternative coupling methods.

David