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 Post subject: Direct conversion receiver tips?
PostPosted: Jun Thu 13, 2019 2:29 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1445
Location: Santa Rosa, CA
I've never designed and built a direct conversion receiver so decided to tackle
this. I've built quite a few regens and curious to see how they compare. Lots of
great info out there already but wondering if anyone here has rolled their
own (from scratch, not from a kit) and can offer some tips?

Planning to add features like AGC, preselector and a Lamb type noise limiter.
It'll be half of a CW QRP rig for the ham bands.
Steve W6SSP

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Thu 13, 2019 6:02 pm 
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Location: Perrysburg, OH, U.S.A.
If you aren't already aware of it for solid state designs you need this book:
Attachment:
Solid State Design for the Radio Amateur.jpg
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Published by the ARRL and authored by Wes Hayward and Doug DeMaw. Unfortunately it's out of print but you can find copies on Amazon for a lot more than the $7.00 cover price. They have several examples of direct conversion circuits along with a wealth of other design information and circuit theory.
John

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Thu 13, 2019 8:16 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
https://ibb.co/ZM3N1T4

You can try this circuit. Using a 74HC4053 digital switch as detector means that the receiver is immune to AM interference as there are no rectifier diodes in the mixer. The local oscillator frequency needs to be 4X the receiving frequency. A DDS VFO can be used here.


Last edited by Dare4444 on Jun Fri 14, 2019 3:05 am, edited 2 times in total.

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Thu 13, 2019 8:18 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
http://www.phonestack.com/farhan/dc40.html

Another circuit using a traditional diode based double balanced mixer and JFET VFO plus buffer to drive the DBM. It is a high performance circuit.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Fri 14, 2019 3:17 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
DC RX using 74HC4066 chip.

https://ibb.co/Lhndgyw

The 4.7uH inductor is a readymade molded choke. I built this circuit few years back and it worked well.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Fri 14, 2019 3:24 am 
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Location: Littleton, MA
OldWireBender wrote:
If you aren't already aware of it for solid state designs you need this book:

This one is even better:
http://www.arrl.org/shop/Experimental-M ... t-Edition/

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Fri 14, 2019 9:20 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
Another direct conversion RX circuit with 74HC4046 mixer and JFET pre-amplifier. The input impedance is high enough for a wire antenna to be used for 40M band.

https://ibb.co/HhCchfc

Broadband transformer is FT50-43 with 10 turns on the primary and 2x3 turns on the secondary.

The JFET can be replaced by a J310.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Fri 14, 2019 10:52 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
Build this circuit in 2014. It has a single balanced mixer balanced by a 100ohm pot. The two IN4148 diodes are matched.

https://ibb.co/Ctm5s9M

No input matching network has been used. Pre-selector was a 1.4uH toroid coil resonated on 40M band with a parallel capacitor. An Arduino based DDS VFO fed approximately 6mW to the mixer RF port. A single LM386 was used for audio amplification.

This simple circuit can be perfected with an input matching network and one more LM386 audio stage can be incorporated. In that case the gain of the first LM386 should be reduced to 20 by removing the 10uF capacitor between pins 1 and 8. This would significantly lower the noise of the receiver.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Sat 15, 2019 12:55 am 
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Joined: Mar Thu 01, 2018 1:30 am
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https://ibb.co/S6sRjSM

DC RX using two J310s.
Input impedance is 50ohm. A low pass filter would be needed. It offers great performance on 40M.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Sat 15, 2019 2:48 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1445
Location: Santa Rosa, CA
Thanks for all of the tips guys. Did not realize how popular DC receivers
are. Got my first breadboard circuit working yesterday. The VFO uses
a BC-221 variable cap/gearbox in a Hartley circuit using a 2N4416A
JFET. The initial range is 9 - 18 MC so I can hit two WWV freqs, 30
and 20m. Mixer is an SBL-1 with a 2N4401 driver/buffer. Audio amp
is an LM386 but need more audio gain. Thats todays project.
Then work will begin on a preselector.
Steve

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Sun 16, 2019 12:08 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
Got schematic?


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Mon 17, 2019 2:06 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1445
Location: Santa Rosa, CA
No schematics yet. This is a new area for me so researching interesting
designs and prototyping them to see how they compare. The version
in the 1988 ARRL handbook which uses phasing (much like vintage
transmitters did) to detect SSB looks interesting even though I'm
primarily interested in CW. May build this one next. I can see the need
for a low noise audio preamp following the mixer and this design uses one.
Steve

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Mon 17, 2019 4:26 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
Yes. The preamp following the mixer will determine the overall noise floor of the receiver. Ashar Farhan in his DC receiver used a transistor preamp. The current through the preamp is 0.5ma.... this gives 50ohm input impedance at its emitter for proper termination of the mixer. It also makes the receiver very sensitive.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Mon 17, 2019 4:35 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2982
Location: Lexington, KY USA
I trust you know that an LM386 is not a low noise device. For the first stages of your audio you are better off with transistors or a low noise op-amp.

Ted


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Tue 18, 2019 12:46 am 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
One more schematic of a DC receiver I found on my old phone. The mixer is terminated by a 51ohm resistor and it directly feeds a low noise op-amp audio amplifier stage. This greatly simplifies the design.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Tue 18, 2019 2:27 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1445
Location: Santa Rosa, CA
Screwed up on the date of the ARRL handbook I referred to earlier. Its the
1996 edition.

Yes, I realize the LM386 if used alone is not a low noise device. Thats why
I'm using an LM387 (only one of the two op amps for now) before it. Oodles of gain
requiring a seperate cast aluminum box. And its billed as a low noise device.

One thing I've noticed in all of the designs I've looked at so far is the lack
of an active preselector. My experience with tube receivers has shown a good
preselector makes a HUGE difference in performance. That would seem to
be especially important with a D-C receiver. This is the next item I'll be exploring.
Steve W6SSP

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Tue 18, 2019 2:50 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 195
For a preselector, build a simple 2n3904 or 2n3866 broadband amplifier with input and output impedance of 50ohm and use a band pass filter at its input. Its output will go to the DBM mixer. Transistor broadband amplifiers typically have 20dB gain.


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Wed 19, 2019 2:15 am 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1445
Location: Santa Rosa, CA
There is a big difference between a preselector, like the RME DB-22A and a broadband
amp. I've experimented with broad band amps for the regens I designed and built
and they all degraded performance.

The design I'm considering for the preselector is an input buffer stage (low
input impedance, high output impedance) followed by a tank circuit
which will tune the band of frequencies I'm interested in followed
by another FET (or MOSFET) stage with a high input impedance. The goal
is to keep the loaded Q on the tank circuit as high as possible.

Many high performance receiver, like the Mackay 3030A and Racal RA-17
use a manual preselector. Others like the R-390 family have it ganged
with the main tuning dial.
Steve

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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Thu 20, 2019 2:18 am 
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Joined: Nov Wed 17, 2010 11:14 pm
Posts: 167
Hi Steve,

Here is an active pre-selector block diagram. It is currently in use in front of a homebrew single conversion receiver which started out as a DC receiver. The pre-selector covers the 160-15 meter band in stepped sequences controlled by a series of relays. The key to the narrow pass band tuning is the use of a SERIES TUNED SINGLE POLE resonator whose loaded Q is controlled by the impedance transformation provided by the transformers. It has a 1 dB BW of less than 700 kHz, minimum gain of 8 dB and maximum gain of 20 dB. The noise figure in all cases is less than 3 dB. IMD is TBD. However, current to date I have seen none.

Correction, pass band gain is closer to 27 dB maximum. Of course tuning the pass band of desired center frequency will directly reduce the insertion gain. The front end switch provides a direct pass thru with no gain, otherwise ~ 9 dB gain. Additional gain and adjustment of center frequency pass band is then possible.

I should also add, the 9 dB gain amplifiers are all transformer feedback type, aka Norton-Anzac developed and are fairly easy to construct. Their attributes are low noise- high IP3- 50 ohm in/out match and if built proper, unconditionally stable. THey cascade quite well.

Alan W4AMV


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 Post subject: Re: Direct conversion receiver tips?
PostPosted: Jun Mon 24, 2019 8:31 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1445
Location: Santa Rosa, CA
Thanks for the suggestion Alan. A different approach that I had planned but
will give it a shot. So far have not been overly impressed with the D-C designs
I have tried. Even when using an external tube preselector. Simple regens
with two stages of audio do better.

Which HP network analyzer are you using? Or is it a spectrum analyzer
with a tracking gen? I was an engineer at HP for a long time in the
division that designed and built these but can't place yours.
Steve W6SSP

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