[HBQRP] Minimalist's QRP E-list - TTL Idea's

Sat Mar 14 15:56:04 EDT 2009

Hi Group!

FYI:  Found a very interesting e-mail on the new Yahoo Group at:

Minimalist_QRP_Transceivers at yahoogroups.com

I hope you will check it out.

I also thought that I would pass along the E-mail as it has a LOT of
interesting links to pages for builders, and some ideas on using
Digital IC's for analog rigs in the OSC. and Mixers and also power amps.

Here's an outright cut and paste of the E-Mail......
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 Using TTL devices to build minimalist QRPp equipment.
    Posted by: "p_e_colla" pcolla at frsf.utn.edu.ar p_e_colla
    Date: Fri Mar 13, 2009 4:33 pm ((PDT))

Hi,

While working on a quadrature detector for an SDR design I'd found the usefulness of oscillators being made out of TTL logic gates, either one or two inverter gates or similar variants using NAND gates.

However the circuits bring significant fight to behave correctly both in stability and specially consistent start-up when used with crystals at 3.5 MHz; at the beginning because of reasons not at all clear to me.

At the end you make it work, but it would be nice to be able to predict better the behavior at design time, like in any other circuit.

I'd found plenty of material in the Internet in this kind of designs at different frequencies (often above 10 Mhz), but material for 3.5 MHz is scarce and using crystals even more. Most designs uses resonators instead which behaves differently and in my case are hard to find.

A very interesting link I found about crystal oscillators design, with some specifics on these made out of TTL gates can be found at (http://www.axtal.com/data/publ/ukw1979_e.pdf) , together with some mail exchanges with Miguel Bartie (PY2OHH) I started to understand the issues and address them in order to have a reliable oscillator.

At the same time I realize the TTL gates has a number of attractive attributes, starting from being cheap, readily available and in general being fairly tolerant to mistakes and abuse during the prototyping phases in terms of tensions and currents in excess. However it is still kind of odd to think on them as useful components for RF work with analog signals so few projects address their use.

The ARRL Handbook of 1997 (Chapter 17) bring a ½ W CW transmitter for 10-15-20 Mts presented by Len Smith (N7KSB) based on a single 74HC240 (octal inverter); the SPRAT magazine also presented several articles from KL7JHM, DL1ZB, G3DOP and G4UOV (issues 10,33,51 and 55) among others.

The approach has many incarnations since them with minor variances such as the one published in the Flying Pigs' Bacon Bits Nov 2000 issue  (http://www.fpqrp.com/BBITS/bb1100.pdf), as well as the design at  (http://www.qsl.net/5z4ft/74hc240qrp.html) among others.

Miguel Bartie (PY2OHH) has many samples on his immensely resourceful web site at http://py2ohh.w2c.com.br

Although the square wave signal seems initially to be very distorted to be useful, fairly simple filters allows to get a reasonable clean signal with ease; at least as clean as in many other minimalist rigs.

Looking at some other "minimalist" designs based on TTL gates I stumble on the transmitter and receiver pair made by AA1TJ called Snowflake  where a single NAND 7400 gate are used as oscillator, buffer and amplifier (http://mjrainey.googlepages.com/Snowflake.pdf), the transmitter itself has few mW of output power. 

But the really interesting piece is the receptor (http://mjrainey.googlepages.com/80mTTLRcvr.pdf). At first glance there is something "odd", and it is that digital gates are used to manage analog signals.

With the aid of the datasheet it become obvious the trick is achieved thru heavy negative feedback which makes the gate operate as a linear amplifier behaving as a mixer and an audio amplifier. At the end a TTL gate is a common mode high gain amplifier, but It's quite a novel way to look at them. Feedback are very critical and specific for the gate technology (HC, LS, CMOS, etc).

Continuing with the search a design called OZON by  Hans Summers G0UPL (http://www.hanssummers.com/radio/ozon/index.htm) is an attempt to pack a minimalist transceiver in yet another  mints package. 

The transmitter part is fairly similar to the original from N7KSB and the receiver is made out of a MiniCircuits SBL-1 based DBM paired with a couple of audio amplification; also a 74HC4066 is used as a simple yet effective T/R system. 

The SBL-1 is a rare beast in my country, and when found is expensive for our terms (USD 15-20) so I look at other alternatives.

Working with SDR circuits made me used to essentially see mixers as an analog switch being commuted at the pace of a local oscillator; the G0UPL approach has already a 74HC4066 (a digitally controlled analog switch) paired with an already available LO signal makes for an invitation to try.

On further search the idea wasn't original at all, Miguel (PY2OHH) has this concept explored in his site where a 4066 is used at different designs as oscillator, buffer and mixer (http://py2ohh.w2c.com.br/trx/4066rx1/4066rx1.htm).

To move all this concepts into work I put together a ¼ W CW transceiver for 3.5 Mhz I'd called Xarma (http://f1.grp.yahoofs.com/v1/cOW6SUWRvgUarftApcdg83GCpf-47fPgMzx3kIzWu6RDf0cLVJwXo2UQrhVt9J6nkomCCh_2iewGA-mf5V3yOy_HYBRRd-Ua/TTL/Xarma_Schematic_v1.9.pdf), the schematic is located in the file area of this forum.

The implementation has a 74HC240 as the oscillator, buffer and PA; a 74HC4066 as a mixer, TX offset and T/R logic and a LM386 in a classical Pixie configuration as the AF amplifier.

I do typically pair minimalist rigs with SDR software (such as the M0KGK transceiver) for an "SDR Augmented" experience taking advantage of the NB, AGC, Filter capabilities and inherent RIT this approach has. 

On the air tests were satisfactory and achieved a performance similar to other minimalist rigs I made (such as the Pixie 2) at similar power levels.

The design is taking lightly on the TTL gates by using +5V instead of the more common approach to near frying them by using +8V and thus sacrificing some output power but simplifying the overall rig design and construction and operation stability.

Although none of the above concepts are a big discovery there are substantially dispersed thru the Internet in several sources, sometime buried in other information, and I thought it might be useful to share my experimentation notes with you all.

73, Pedro LU7HZ
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I hope that its of interest to you! 

73 - Arnie KA0NCR