Direct Conversion Receivers - RF Bandpass Filters

Today I'm turning my attention to the RF bandpass filter used between the aerial and the product detector. If you see a circuit that just uses a single tuned circuit for filtering then move on. Really. The maths and software for working out values has existed for so long that there is no excuse for single tuned circuits.

So let's move on to considering double versus triple tuned circuits. It's very easy to use filter software like Elsie, AADE or Iowa Hills. My favourite is the Iowa Hills software. It does 80% of what I want with the least effort so I always start with this software.However, I'll step you through how I made an error and suffered from a deaf receiver till I worked it out.

This first part was done in Elsie but it was not the fault of the software. Rather, it was a case of garbage in, garbage out. Here is a double band pass filter for 40m. Since it is a 300KHz segment we will use that as the bandwidth.

I like it, but perhaps a triple band pass filter will look better?

I'm getting excited. Let's compare the two responses on the same graph to see what we trade-off for the extra selectivity:

Okay, we give up 2dB of loss in the passband for much better selectivity. Sounds good to me. Or is it? At this point I was pulling my hair out with a deaf receiver. How did that happen? Quite apart from using by mistake the wrong value parts when I built the filter, it transpires that the curves above were designed around inductors with an unloaded Q (Qu) of 200. What happens when we use a more realistic Qu of 70 for the inductors? We suffer an extra 5dB of loss in the case of the triple band pass filter with this bandwidth.

Which gives us almost 8dB of loss before the product detector. In urban areas that may not be an issue if you have high noise levels. But in my location 8dB sounds excessive given the low noise levels I enjoy. And this extra loss compounded a problem I had which we will discuss in another post concerning low noise op amps.

I learned my lesson. Check your assumptions before you build. In practice I start with a bandwidth of 10% of the centre frequency, say 715KHz in the case of 40m. I then adjust the bandwidth and terminating impedances until I get some standard capacitor values.

You can iterate away a lot of time if you're not careful. My favourite package, the Iowa Hills Filter Designer package, is well suited to this task. After just a few clicks I get the following circuit and response:

Which all looks really nice. But no loss? That doesn't make sense. Remember to check the real parts box! The resulting loss is 1.5dB. That was so easy I repeat the exercise for a triple bandpass filter. I get a loss of approximately 2.1dB. At the third harmonic of the local oscillator, or 21MHz, the 2 pole filter has an attenuation of 15dB. In contrast, the 3 pole filter has an attenuation of 42dB. I don't want to be hearing stations on 21MHz when I'm tuning around on 7MHz. Given the improved selectivity of the triple band pass filter over the double, I elected to go with the triple despite the slightly higher loss. Finally, I tweak the design by broadening the response while watching the loss at 7MHz and 21Mhz. I end up with less loss than the 2 pole filter but better attenuation at 21MHz.

All of which took me less time to do than write! Here are the two filters compared:

Which is all fine so far. We have standard capacitor values. Inductors are always harder to source. If your junkbox has variable inductors of a nominal 600nH then you're in business. The Q of these is likely to be around 70. Coilcraft make a really nice series of 10mm vairable inductors which would be ideal and with Qu in the mid 70's match what I assumed in the software. (Coilcraft, if you're reading this I'd love some of your stuff but it's so damn expensive in Australia that I can't afford it!)

So off to the junkbox. What? No 600nH inductors.

Lots of recovered variable inductors around 3uH though. Back into the software. Hmnnn. 3uH is too large a value for the shunt LC tank to work. I try the series option just in case. Viola! I get a practical design with losses comparable to the version discussed above and a suggested 90dB of attenuation at 21MHz. I really doubt I can achieve that in practice without serious shielding but it should go a long way towards attenuating signals on harmonics of the local oscillator. Of course, there is no free lunch here. I have less attenuation of signals below 40m now. Of concern is a nearby 50KW AM broadcast signal on 720kHz breaking through but to date I haven't noticed any such problems.

Final 3 Pole 40m Bandpass Filter - Series versus Parallel configuration

I found the triple bandpass filter easy to peak using a signal generator on 7150kHz and a local oscillator or 7149kHz. I tuned for maximum audio after the audio low pass filter, the subject of the next post.

I hope you're inspired to download the Iowa Hills software and start rummaging through your junkbox for variable inductors.

73's
Richard VK6TT