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Old 4th Feb 2023, 1:58 pm   #1
Malcolm T
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Location: Lugo, Spain
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Default HF Direct conversion on 10mts Rx

Can someone steer me toward DC receiver design ie a simple build please using general purpose transistors, no FET,s . or IC,s. Has anyone built one , there seems to be a lot of resurgence on DC receivers due to the simplicity for qrp portable work.
The higher HF bands are of interest 10mt band especially.
Ta very much.
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Old 8th Feb 2023, 11:34 am   #2
G3PIJpeter
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Default Re: HF Direct conversion on 10mts Rx

Have a look at
https://miscdotgeek.com/building-a-d...ceiver-part-2/
It uses 2N2222 bipolars as your requirements. This design is for 40 metres - adjust the input filter for 10 meteres and inject 28 MHz.
Peter G3PIJ
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Old 8th Feb 2023, 11:41 am   #3
G3PIJpeter
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Default Re: HF Direct conversion on 10mts Rx

. . . and there's this one - again, amend resonant circuits for your band of choice
https://jbcs.co.za/ham_radio/dc_rx_original.php
Building a stable VFO for 28 MHz to copy CW and SSB may present some problems though.
Peter
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Old 9th Feb 2023, 5:20 am   #4
tanveerriaz
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Default Re: HF Direct conversion on 10mts Rx

search in google "V. Polyakov (RA3AAE) Direct-conversion receiver"

se
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Old 9th Feb 2023, 6:57 am   #5
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Default Re: HF Direct conversion on 10mts Rx

Have a look at the American QRP-ARCI club's "QRP Quarterly" magazine, or the British G-QRP Club's magazine "Sprat". Both have published a great many direct conversion receiver designs over the years. Both organisations, despite their geographical names aggregate material from a worldwide membership.

The G-QRP club sells an archive of all copies of Sprat from inception almost to date on CD ROM, or on USB stick for a reasonable price. This may be the best place for mining DC RX projects.

Switching to a technical viewpoint; the higher HF bands have opened up and there is a lot of activity, so I can see how the interest has arisen.

A direct conversion receiver doesn't need an input filter at the RF input, but one helps to reduce the total power of signals hitting the mixer. If you're hunting weak signals, this will reduce intermodulation and harmonic products being created from strong lower frequency signals.

Most DC receiver designs are for the lower frequency bands where noise floors are high, and the receiver does not need to be particularly low noise, so it's usual to not have any amplification in the RF path before the mixer, even with a lossy mixer like a diode-ring type (typically 7dB loss). But on the 10m band, natural noise floors are much lower and having an RF pre-amplifier will help. Having an RF filter will help protect both preamp and mixer from out of band signals. RF preamp designs intended for normal superhet receivers work just as well in front of direct conversion receivers.

Coming to the mixer, there are many choices. Half-balanced and fully-balanced diode types are popular. The SBL-1 and MD108 packaged units are 4 diodes and two transformers in a screening can. They can not just work well at 10m, but can go well into the VHF spectrum. Even the cheap little NE602 IC mixer is capable of going far above the 10m band. So the mixer section need not be different to those of the more usual 80m and 40m designs.

Looking at the audio amplifiers, this area is also affected by the lower noise floor of the 10m band, and as most DC receivers have little gain (usually net loss) ahead of the audio amp, then having a low noise amplifier is beneficial.

I've left the Local Oscillator until last because this is where the choice of 10m creates the biggest problem. It's one of the main reasons why home-made DC receiver articles favour the lower frequency bands.

To receive SSB and CW signals needs a stable local oscillator, the amount of drift needing to be small compared to the audio frequencies being heard. However, the amount of drift in the LO and the difficulty in reducing it, scales up with the frequency of the band being received.

If you're wanting to make a simple receiver without fancy crystal oscillator-mixer-VFO oscillator systems, then you'll find making a VFO stable enough for use directly in the 10m band can be challenging.. Finding good quality variable capacitors, gearboxes and arranging mechanical rigidity will be the main task. Yes, you can keep your hand on the tuning and keep correcting drift by ear. but this soon becomes annoying.

A possibility is to get a mechanical assembly from a piece of high quality test gear where the hard work has already been done. George Burt GM3OXX is legendary in QRP circles with well over 300 countries confirmed with 1 watt transmitter power. He used a homebrew transceiver with a high frequency VFO, but the tuning capacitor, gearbox were an integrated unit from a scrapped BC221 wavemeter. The ceramic coil former came from the same source, but was re-wound.

Hope this helps you judge the different designs you'll come across, and may help you to change a low frequency design into a 10m one.

David
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