Saturday, 7 November 2015

Channel Separation in Parallel IF Receivers

The "Parallel IF" scheme, developed in this receiver, described in the January, 2015 edition of RadCom and described and demonstrated at FDIM 2015, steers the incoming signal in a superhet receiver through one of two Parallel IF filters. These two filters allow the rig to switch between receiving bandwidths appropriate to (e.g.) SSB and CW operation, just by change of IF frequency.

Whilst the theoretical separation between the two IF paths is high, any practical realisation will have only a finite degree of isolation - there will be some breakthrough on the "unintended" IF path when the other is in use.

This was noted in the RadCom article, which states: "Very little of the signal from the tuned station passes through the other, ‘unintended’ IF filter (whose response is drastically attenuated 2MHz away from its passband)." It has also been my experience in using the method - pulling the "operational" filter out of circuit (easy to do with my plug-in modular filters) effectively muted the receiver. However, I decided it was important to make some measurements to test this separation in a practical Parallel IF rig.

I used the plug-in BITX as the test platform, and applied a 7.03 MHz signal from a DDS, via an attenuator, directly to the antenna input...

I tuned the radio so as to achieve an output at 1 kHz and monitored the output voltage on a PC analyser called "Visual Analyser". I coupled the receiver to the PC soundcard using an isolating transformer, to reduce problems with mains/line hum.

Here's the system output as measured through the CW IF filter (which uses a 12 MHz IF path)...

I disconnected the input to the 12 MHz CW filter, leaving the IF set at 12 MHz and the other (10 MHz) filter in place. The input was still applied to the receiver. The response changed...

Note that the signal is now 48 dB down on the original - down in the noise floor. It is inaudible.

When I switched to the SSB IF filter (at 10 MHz), the signal re-appeared...

Note that it is matched closely to the level achieved through the other IF path (it is roughly 1.5 dB down), suggesting that the gains of the paths are well-matched (as confirmed by listening). However, the wider bandwidth of the SSB filter has admitted more noise and increased the level of harmonics of the 1 kHz signal (possibly as the wider filter bandwidth causes the stages after the filter to be driven harder).

Interestingly - and quite by accident - I discovered that  separation between the two IF paths was poorer if I completely removed one of the filters. Here's the response when the system is set to use the 12 MHz) CW filter, but that filter is removed...

The "breakthough", coming past the remaining 10 MHz SSB filter, is now only 36 dB down on the original level. Remember (from above) that when the filter's input was open-circuited, the residual was 48 dB down. Here, when the filter is completely removed from the circuit, the performance is poorer.

These measurements have confirmed my subjective impression - derived from experience - that a practical Parallel IF receiver (even one made within the poverty of my construction skills) can achieve channel separation entirely adequate to its intended use. It can also match the in-band gain of the two paths.

What a relief!

... -.- de m0xpd

1 comment:

  1. Very nice! I've just started experimenting with some arduino-controlled DDS VFOs after 10 years off the air. Data like this is super helpful as a guidepost for those of us who are just starting out and experimenting. 73