40m DC receiver (2)

After my previous attempt at building a 40m DC receiver failed (I received nothing but noise while at my Kenwood HF receiver, plenty of signals were heard), I made some changes to improve the sensitivity and to add some features that were desperately needed..

After I connected the antenna to the receiver, hooked-up my function generator as VFO I received ….zilch nothing. L


The next part was the inclusion of an AF filter. Currently, the receiver bandwidth was only limited by R6/7 and C4. This RC filter sets the cut-of frequency at circa 15 kHz. To limit the bandwidth to a more common bandwidth an 5-pole elliptical filter was included.

The filter has a cut-ff frequency of 2.8Khz. The Insertion Loss (IL) is circa -7.9dB. This primarily caused by the resistors R1 and R6 and the internal resistance of the inductors 91 ohm. (Unfortunately, you cannot just change these resistors for they are intended for proper termination of filter and ensures a nice flat passband band and sharp transition.)

The filter was simulated in LTSpice. Above you see the simulation result. Left in dB and log Hz, on the right linear mV on the vertical axis and liner frequency scale on the X-axis as well. Below is the result of the real product.

Vertical 200mV/div, horizontal 500Hz/div, starting with 500Hz on the left. The output of the function generator was 3Vtt and set to 600 ohm. The measured insertion loss is thus -7.4dB.

Finally the filter was measured with my soundcard audio interface and the audiotester V3.0 software(which is highly recommended by the way!). You can see the result below. Nice to see the outcome of all the tests are in line with each other.

40m DC Receiver Audio Filter fc =2.75kHz

Input Band-pass filter

The next step is an input band pass filter. This filter has several functions. It filters the incoming signal so that only the transmissions in the range of 7.0 – 7.3 Mhz are passed through. This limits the chance of IMD distortion in RF amp and the following stages. It functions as impedance transformer from the standard 50 ohms input impedance to the 10k input impedance of the RF amplifier at the same time it provides circa 16dB of voltage gain!

The required wire length was calculated using the excellent toroid calculator on toroids.info. For 32 turns it indicates a length of 60cm including 2 2.5cm pigtails. Usually, I’ll take the wire a bit longer so I can add a couple of turns. It is easier to remove some wire then to add a bit later on or to completely rewind the coil. The extra length helps me to tune the coil to it’s desired value.

Update 19-04-2022
Last week I did some measurements because I still have problems getting the receiver to work properly. I found that the current consumption was excessive (close to 95mA, while around 25mA was expected). Turns out the culprit was the 74LS132, when disconnected, the current consumption felt back to normal.
I directly connected the function generator to the receiver. The RF port was set to fRF = 7.000 Mhz, 2 mVpp. The local oscillator port was connected directly to the switches and was set to fLO = 7.001 Mhz and 5V TTL. Because there was severe clipping at the output of the op-amp. the output was taken directly from the collector of Q2.

The measured output was 5.2 div x 100 mV/div = 520 mVtt. So the gain from the input up to the collector of Q3 = 48dB. The op-amp, once added, will add another 40dB of gain. The LM386 set to a gain of 200x, will ad another 46dB. This brings the total gain to 134 dB. Plenty enough to copy the weakest ether signals.

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