Gain distribution in DC receivers

For an upcoming 40m (7Mhz) DC receiver that I am working on, I wanted to know the necessary gain. After building the first prototype, the output was kind of quit. So the question came up, how much gain do I really need for a DC receiver? Back of the envelope calculation : to get from 1uV to 1V is a voltage gain 1*10E+6 or 120dB. 1uV corresponds with S3. S1 is equal to 0.2uV. Thus the voltage gain must 5*10E+6 or 134dB. So that’s is theory. Now let’s have a look at some real-world DC receivers.

DC-80 by Drew Diamond VK3XU

The DC-80, an 80m Direct Conversion Receiver, is a straightforward receiver with a very common topology. The blocks that can be distinguished are: input BPF, RF amp, NE602 mixer, AF amplifier and power amp. As such it is an ideal candidate to calculate the gains as there are many published receivers with a common architecture.

The input filter (L1, L2, C1-C5) starts with a step-up transformer with a ratio of 3 turns : 41 turns. it has thus a voltage gain of 41/3 is 13.6x which equals to a gain of 22.7dB. The filter is terminated with 3k3 resistor (R1). Assuming the transformer L1 has the perfect impedance tranformer ratio, the gain would be √(3300/50) = 8.1x = 18.2dB. Throwing it in LTSpice gives a gain of 19.8dB. All figures are pretty close together. We will choose 20dB for each of calculation 😉

The RF amp is build around a dual-gate MOSFET MFE131 (=40673), a once popular choice of active device for gain controlled RF amplifier and mixer. Alas, it has become obsolete. The datasheet list the Forward Transfer Admittance to be within 8 and 20 mA/V. We will choose the middle road 14mA/V. For AC the DG-MOSFET is configured as common source amplifier because the source is connected to ground via capacitor C8. The resistor connected to drain is determined by transformer T1 and the input impedance of the NE602 mixer, 1500Ω. The transformer is trfilair wound and has a turns ratio of 1:2 and thus an impedance ratio of 1:4. The load presented to the drain of Q1 is thus 1500/4 = 375Ω. The gain will thus be 375Ω * 14mA/V = 5.25 == 14 dB.

Drew has choosen the NE602 as an active mixer device. The datasheet lists the gain as min 14dB and a typical value of 17 dB.

All the audio amplification is handled by op-amp A1. It is set-up as a differential amplifier, with a gain of (A-B) x R9/R7. If A and B are equal but with opposite value the gain becomes 2 x A x R9/R7. However, since the input impedance of both sides are totally different (inverting input < 2k2, non-inverting input 220kΩ + 2.2kΩ) and the output impedance of the NE602 cannot be ignored (1.5kΩ), the actual gain will be more like R9 / R7 = 100x = 40 dB.

For maximum sensitivity the AF potmeter is set to no attenuation and thus 0 dB loss.

The audio amplifier is build around the common LM386. Gain pins 1 and 8 are left open, thus the gain is set to the default value of 20x = 26 dB.

So the total gain in this receiver : 20dB (LPF), 14 dB (RF amp), 17 dB (NE602), 40 dB (AF amp), 26 dB (AF amp) = 117 dB.

Multiband DC Receiver, Hannes Coetzee ZS6BZP

This receiver was published in Electronics World February 1998 and might be one of the first published receivers that is using the the 74HC4066 as digital switch mode mixer. The architecture is much like the DC-80, with a couple of small but significant changes. Primary difference is that Hannes has been focusing on proper impedance matching throughout his design. The receiver blocks are, in order of appearance, input BPF, mixer and diplexer, AF amp , AF filter and power amp.

The receiver has been designed fo 7MHZ, 14 Mhz, 21 Mhz and 28 Mhz amateur bands. To service these bands the receiver is quipped with switchable input filters. The input filters has been design in such a way by ZS6BZP that for each band they should have an insertion loss of ca. -1 dB.

The mixer is build around a 74HCT4066 quad SPST switch. This configuration requires a much lower power drive signal. The “ON”resistance of those switches is around 50Ω. The mixer is followed by a diplexer build around C3, R2, L1, L2, T1, R1, C1 and C2. The input impedance of this configuration is 50Ω as well. The resulting output can expected to be halved (-6 dB). Hannes quotes an insertion loss of -7dB.

The first AF amplifier starts with a step-up transformer on a RM6T35 core with a winding ratio of 100:2000. This will create a passive gain of 20x = 26 dB. This is then directly followed by inverting op-amp around OP27 (IC1) with a gain of 470k/ 18k = 28.3dB.

This amplifier is followed by a switchable passive elliptical LC audio filter with a cut-off frequency of either 2.4kHz (SSB) or 800Hz (CW). The input impedance of this filter is 500Ω and the insertion loss is -6dB.

After the filter one more AF amplifier is added with a gain (1+33k/220)= 43.6dB. .

The AF power amp is build around a TDA2030 and is configured for a gain 36.8dB.

So the total gain of this receiver is -1dB (RF input filter), -7dB (mixer and diplexer), 26dB (AF trafo), 28.3dB (1st AF amp), -6dB (AF filter), 43.6dB (2nd AF amp), 36.8dB (power amp)= 120.7dB.

DC Receiver for 80m and 40m, PA3HDF

Another simple and straightforward DC receiver for the 3.5Mhz and 7 Mhz bands is designed by OM PA3HDF. “Simple” not as in small number of components but as in a design that uses easy to obtain components and straightforward but clever circuits without any complicating tricks. Not depending on tricky to set-up circuits. The receiver chain is much like the previous receivers. No input BPF this time, JFET RF amplifier, 74HC4066 mixer, AF amplifier, selective AF amplifier and power amplifier.

The gain for the RF input amplifier, balance transformer, mixer and 1K resistor is easiest calculated if taken as a whole. The 74HC4066 DPDT switch with a certain on resistance. The 1K resistance shows as a 10 t / 6t *1k =1k66 resistance to the drain of the JFET. If we presume an admittance of circa 3.5mA/V. The overall gain is then 1.6kΩ * 3.5mA/V = 5.62x = 15 dB. Note that because of the high termination resistance of the mixer (1kΩ), the ON resistance of the switches play a smaller role in this set-up.

The first audio amplifier is set-up as a “difference” amplifier. (See for review of this set-up in the DC-80 receiver). The gain is 100x = 40dB.

The selective audio amplifier is configured as band pass filter with a center frequency of 700Hz and a gain of 38dB (determined by simulation).

The audio amplifier is set-up (again) around an LM386 amplifier with a gain for 200x =46dB.

The total gain is then 15 dB (RF amp, mixer) , 40dB (first AF amp), 38 dB (selective audio amp), 46 dB( final amp) is a whopping 139dB. This is quit a bit more then the other receivers. But which might be very much needed considering that the design PA3HDF is intended to be connected to a (small) loop antenna which has a low conversion gain.


BlockDC-80 / VK3XUMultiband / ZZ6BZPLoop DC / PA3HDF
BPF20 dB-1 dB
RF Amp14 dB15 dB
Mixer17 dB-7 dB
AF amp 1st40 dB54.3 dB40 dB
AF filter-6 dB38 dB
AF Amp 2nd43.6 dB
Pwr Amp26 dB36.8 dB46 dB
Total117 dB120.7 dB139 dB

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