Heathkit HW-7 part 5 – output filter

Looking at the original 1972 manual for the Heathkit HW-7 the specification for spurious and harmonics is only 25 dB down on the transmit carrier. Modern commercially made amateur radio equipment is designed to meet the ITU specification for HF amateur radio equipment which is 43 + 10 log (PEP), or 50 dB, whichever is less stringent.

Testing the transmit output on a spectrum analyzer showed that they were not generous with the specifications the second and third harmonics were only just 25 dB down.

At Cross Country Wireless one of our filter products for the professional market is a bandpass filter that uses a combination of cascaded high pass and low pass filters. In the filter textbooks this is the filter that’s mentioned in the first few paragraphs on bandpass filters but is then rejected as being far too difficult to model mathematically. As long as the high and low pass filters are well spaced in cutoff frequency we can design and manufacture some very effective filters.

The photograph shows a bandpass filter designed for the HW-7 using this technique. The high pass filter cuts off at 6.5 MHz and the high pass filter cuts off at 25 MHz. The PCB board is the one we use in the filters. What is not obvious is that there is a plated through hole at each of the crossing points on the board connected to an identical grid on the other side. When the board is bolted into an enclosure the grid is very effectively RF grounded to the enclosure. BNC connectors were fitted for testing on the network analyzer.

Obviously as it’s not being built for a paying customer I’m using a board that’s been previously used to build a prototype hence all the excess solder on the board. What’s the old saying…a cobbler’s son is always the worst shod!

The bandpass filter does work well. The network analyzer display is covering from 3 to 53 MHz (you can see the 50 MHz marker on the right of the display). The vertical scale is 10 dB per box so at 3.5 MHz the filter is 68 dB down. In the medium wave band the attenuation is even higher. On another transceiver the strongest local AM broadcast station on 1458 kHz cannot even be detected in SSB mode with the filter in.

For the transmit harmonics the bandpass filter cuts off well and it will certainly help the HW-7 meet the modern spurious and harmonic specification. The return loss (VSWR) curve could be better but it would need more time to align it (what did I say earlier about the cobbler’s son?).

It’s all very well quoting specifications and dB levels etc, etc but let’s see if I can give a visual aid to what the filter can do on receive.

Just behind the workshop we have a 15m vertical wire antenna suspended by a tree. This uses a 9:1 current balun to give a rough match to 50 ohms. Here is the antenna fed into a 50 MHz oscilloscope just as the front end preselector circuit in the HW-7 would see it. The oscilloscope is set to 20 mV per box on the vertical scale so the peaks are filling the screen giving 200 mV peak to peak. Yes, that’s 0.2 VOLTS peak to peak!

Putting the bandpass filter in circuit the signal level drops to 10 mV peak to peak. It’s still a high level but any receiver never mind the HW-7 only stands a chance of working once most of the unwanted RF energy from the antenna is filtered out.

One of the reasons I’ve added this filter is that eventually when it’s completed I’d like to take the HW-7 on a SOTA Summits on the Air activation up Winter Hill. Winter Hill has several transmitter masts including the main TV, FM and DAB transmitter mast with megawatts of effective radiated power coming from it.

I often take prototype products up there for testing on the basis that it they work up there they should work anywhere.

Heathkit HW-7 part 4 – receiver filter

Now that the Heathkit HW-7 is working close to the original specification with all the 45 year old carbon resistors changed for 2019 metal film resistors I can now look at some modifications to bring it up to modern standards.

The first thing I want to look at is the receiver filter. The HW-7 has a simple LC low pass filter immediately after the 40673 mixer so lets see how it performs.

To do this I fed a wideband noise source into the antenna input suitably attenuated so that it didn’t overload the receiver. I then fed the audio from the headphone socket into an external sound card connected to my Windows laptop. I then opened the Spectrum Lab audio analysis program to look at the filter bandwidth and response.

The existing filter is fairly flat from 200 to 1200 Hz at the -6 dB points with the peak at 1000 Hz. It’s not a bad filter for such a simple design as the stopband from 3500 Hz is 50 dB down.

It’s probably a bit wide by modern standards and later I want to modify the transceiver so that it has a correct 750 Hz offset between transmit and receive so can the filter be modified to sharpen it up?

I modeled the existing filter in the Elsie filter program and came up with some capacitor value changes that reduced the loss between the mixer and audio amplifier by 5 dB and sharpened the filter. The peak of the filter is now at 750 Hz and the -6 dB points are at 400 and 1000 Hz. The stopband figure remains the same at 50 dB down.

The changes look very subtle on the display as the 5 dB reduction in loss tends to hide the changes but it does sound a lot better listening to CW on air.

Only three capacitors were changed, C5 is now 0.47 uF from 0.22 uF, C9 is now 0.47 uF from 0.1 uF and the coupling capacitor C52 is now 0.22 uF from 0.05 uF.

I did try experimenting by changing the inductor bypass capacitor C7 from 0.01 uF to 0.1 uF. This gave a really sharp cutoff above 1000 Hz but the stopband figure reduced from 50 dB to only 30 dB so while it would sound great working weak signals at sunspot minimum it would fail listening to a band full of strong contest signals.

With simple filters like this it’s tricky to get the right balance but you can get close with modern computer modeling and measurement tools.