[Radio Wrangler]

Each of them has only a single crystal, and so when that crystal is in-play, there will be a single-pole filter response. Changing the loading to change the Q will adjust the -3dB bandwidth, but the skirts will also change in proportion, until you get far enough out on the skirs for the bandwidths of the resonators in the IFTs to have some effect. A single pole response will always be bullet-nosed and the slopes will be asymptotic to a fall of 20dB per decade (of frequency difference from the centre)

The phasing capacitor of the HRO front panel is there on the AR88, but inside, on the AR88 deck. It's primary purpose is to correct the response for the C-nought parameter of the crystal... this is essentially the capacitance of the case and electrodes of the quartz unit.

Look at the IFT driving the crystal and you see the secondary has a tap. The tap goes to RF ground and has only the AGC voltage on it, for biasing the subsequent valve. The ends of the transformer therefore have equal and opposite signal voltages on them. One end drives the crystal, and the other drives the 'phasing' capacitor. The far ends of the crystal and of the phasing capacitor are joined.

This makes a bridge circuit, and when the phasing trimmer is adjusted to equal the crystal's C-zero capacitance, the currents through the phasing trimmer and through the C-zero component of the crystal will cancel.

Quartz crystals normally exhibit both series and parallel resonance modes, with similar pairs for all the overtone modes, and also for most spurious resonances. Cancelling C-zero this way removes the cause of the parallel resonance modes, leaving us with just the series mode resonances.

So a crystal alone as a series-pass element into some load resistance will normally give a sharp series mode peak with, slightly higher in frequency, a sharp parallel mode null.

The null distorts the skirts of the series mode peak.

When C-zero is nuilled by a phasing capacitor in proper adjustment, the skirts of the series resonance become symmetrical.

This is what goes on in the AR88. Whoever aligns the IF sets the phasing trimmer for good symmetry and leaves it.

The HRO is a bit cleverer than the AR88, the phasing trimmer is brought out to the front panel as a user control. Artful CW operators have found that a null which they can move around gives an added ability to substantially reduce nuisance signals close to the one they are trying to copy.

I've never had a Hammerlund, so I can't comment from an operating viewpoint, but it looks very similar. The Hammarlund circuit switches different loading resistors in, while the AR88 has only one loading resistor, but by switching the output end of the crystal between the top of the resonator sporting the load resistor and two different taps down the resonator inductor it performs the same function in loading the crystal into different resistive impedances. Here the AR88 could have used the voltage step-up to drive the grid of the following valve, but they stuck with the output end of the crystal. This could have increased gain a little on the narrower bandwidth settings, compensating for the increased loss in the crystal. This is something you need to do with variable bandwidth filters such as those in spectrum analysers where you also have to worry about the temperature coefficient of the loss varying with selected bandwidth. I think the AR88 was done this way to avoid still further complexity in the switching, as more trimmers would need to be switched to keep the peaks in the same place for all bandwidths.

Hammarlund's use of a differential phasing capacitor rather than a plain trimmer looks to be a way to keep the various resonators from being pulled as much when the phasing control is used. The AR88 with fixed phasing doesn't need this, the HRO just lives with it. I think it's a neat way to reduce an unwanted effect (reduce not remove) in a set with phasing as a front panel knob.

While it's interesting comparing these three approaches, there doesn't seem to be any driving need to convert an AR88 ot HRO into an SP600. We live in an age where SP600s are affordable, so we can just get one of those to restore.

If filter performance really is a problem, then these sets only deploy a single crystal response, and that only on bandwidths too narrow for telephony. What is needed is a filter of telephony sort of width, but with a flatter top and better shpe factor down into the skirts. This is a portrait of a multi-pole filter. So, if you want to listen to AM or SSB with a crystal filter, then you need multiple crystals.

Those crystals can be deployed in either a lattice architecture, or in a ladder architecture. The ladder has an advantage in needing a group of similar crystals, while for a lattice they all have to be made carefully offset to each other (You can hear the cash registers chiming!) However, ladder structures become difficult for wider filters on lower centre frequencies. 455kHz is too low for easy AM or SSB bandwidth ladders, so it's lattice territory.

The likes of ICOM, Yaesu etc flog option filters for their transceivers. Some transceivers have IFs at both 9-ish MHz and 455kHz in schemes to allow variable bandwidth by offsetting LOs and sliding filters across each other. If you opt for more filters, on the poshest models, you found the 455kHz filter was three or four times the price of the one at 9MHz.

So, if you feel hadicapped by the selectivity of an HRO, AR88, CR100. SP600 etc, etc. Then the next step that really delivers any improvement is to a set with multipole crystal filters. Multipole ceramic filters can also be better in this way, but they were mainly used on cost-competitive hobbyist receivers. Collins' mechanical filters did the job for governmental and military jobs.

Ships' main receivers were pushed into multipole filters, and there are namy which had switched block crystal filters at 2MHz or 1.4MHz. A lot of 1.4MHz filters came on the market in the 1980s as various firms and agencies dumped spares stock. I collected myself a fair selection of a few different CW bandwidths, RTTY ones, USB and LSB as well as a few different AM bandwidths. So if I want to do a homebrew receiver, I'm covered. They come in useful when I want to add LSB to a military set with USB only.

Back in the sixties, you had whatever set you could afford, and if it didn't do what you wanted, you modified it. Nowadays you may want to question your starting point and maybe choose a different receiver. It all depends if you want the end-goal, or an interesting journey.

From a point of circuitry, there is not a lot to choose between the filtering of these three receivers. Each can be set up to give responses very similar to the others. The AR88 lacks phasing as an operator control, but that is only of value for CW reception under crowded conditions.

David