Full-Range IF Selectivity; The Hammarlund Variable Crystal Filter

[ms660]

The HRO-5A1 had switched selectivity settings using capacitors, response curves also shown in the manual:

http://www.tubebooks.org/Books/Atwoo...onalHRO5A1.pdf

Lawrence.

[Bazz4CQJ]

Thanks for that Lawrence. Especially after your comment that you liked the selectivity control of your Hammarlund, and more so than your AR88, I think I'm going to try install a version of the Ham. system in the HRO. As previously commented, Ham. used that system on just about everything the made from '38 onwards, up to and including the SP-600, where they made one very small change to it.

B

[Terry_VK5TM]

Trolling though files on my PC today, I came across the manual for the DL-QRP-AG Speaky CW SSB Transceiver radio kit.

This kit uses an adjustable bandwidth crystal filter based on 8MHz xtals and pin diodes.

Bandwidth variable between ~2.5kHz and 500Hz, page 6 of the pdf attached below.

Might give you some more useful information.

As an aside, for those aware of the DL-QRP-AG Project shop - it has now closed due to various factors including high costs, unavailability of components and falling sales.

[Bazz4CQJ]

That's an interesting and very modern design Terry; maybe that could be my next project?

B

[Terry_VK5TM]

Just to add to your collection of projects, the attached schematic snatch is of the variable bandwidth filter in the TX path of the Yeasu FT290 2m transceiver.

It switches between CW and SSB bandwidths.

[Bazz4CQJ]

Ooh, I have an FT290 on the shelf behind me. Long time since it was last switched on .

B

[Radio Wrangler]

On that filter circuit, the variable elements, the varactor diodes are arranged as one tuning the centre frequency of eack of its resonators. They, operated together will tune the centre frequency of the whole filter.

The bandwidth of the filter is set principally by the coupling capacitors C67, C72 and C75, and the coupling coefficient in each transformer structure, along with the source impedance, and the load impedance (transformed by C78)

It's a classic coupled-resonator style of four poles. R57 may not look to do much, as it only carries reverse-biased diode leakage currents, but it's been put there to block RF leaking around the filter via the common tuning line.

The bandwidth will change a little as it tunes over the 2m band, it will tend to keep the bandwidth as a fixed percentage of the centre frequency.

The FT290 as most multimode transceivers acts as a superhet on transmit, in reverse of the receiver structure, so SSB is created at the set's IF frequency, and then up-converted to the chosen RF frequency (using the same synthesised LO as the receiver) The transmit mixer will create an image offset be twice the IF from the wanted signal. The output stages are fairly broadband and to meet the spurious emission requirements that some countries apply to commercially made amateur gear, a decent filter is needed. This is that filter. It may also be switched on some sets to be used as a preselector in the RF section on receive.

That middle coupling capacitor at 0.5 pf shows they are trying to keep the filter narrow, but the can't make it too narrow or else it will need either trimmer capacitors to tweak up the response on individual units, or else pots to do the same via the varactors. Twiddlers cost money. Adjusting them during manufacture costs a lot more. So this is an adjustment-free design.

David

[Pamphonica]

As an aside to this fascinating discussion, I see from the circuit diagram that the differential cap has to have the rotor and shaft floating, since one side has to be earthed. The few differential caps I have are all conventional, with earthed shaft. With quite small capacitances involved, doesn't this lead to a few hand capacitance effects when operated, unless a long insulating shaft is used with an insulated capacitor mount? Or have I missed something?

[Bazz4CQJ]

Originally Posted by Pamphonica

As an aside to this fascinating discussion, I see from the circuit diagram that the differential cap has to have the rotor and shaft floating, since one side has to be earthed. The few differential caps I have are all conventional, with earthed shaft. With quite small capacitances involved, doesn't this lead to a few hand capacitance effects when operated, unless a long insulating shaft is used with an insulated capacitor mount? Or have I missed something?

No, I think that's all correct. However much the same is true about the phasing capacitor used in the HRO (and other sets) the rotor is floating and indeed the answer there is a longish insulated shaft emerging from the metal screening box and then the front panel. The Hammarlund filter slots in to the HRO, re-cycling much of what is already there.

B