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Initial Ideas

A project log for K8TRC Digital SDR

A software defined radio transceiver designed for digital modes, primarily WSPR and FT8 on HF.

trevor-rh-clarkeTrevor R.H. Clarke 03/26/2019 at 19:030 Comments

I've been wanting a small, low power, portable radio for working digital modes. Given the vast array of low power MMICs and other ICs available these days there must be some gold to be mined digging through manufacturer's product lists. My idea is to use low cost, modern parts, many of which are available for free in small quantities via sample requests.

As a starting point for requirements I've come up with the following list. The radio will likely support a much wider array of modes and frequencies, but this is the core requirements.

  1. Support at a minimum, FT8 and WSPR modes for transmit and receive. The heavy lifting will initially be on a PC but I'd like to support fully autonomous use.
  2. Support at least 40m, 30m, and 20m HAM bands but likely expand to 17m and 15m initially. Full HF, VHF, and 70cm UHF support would be great, likely changing out the front end band filters. I.E. not supporting all simultaneously, but allowing for a component change.
  3. Low enough power use to run off a reasonably sized battery pack for a reasonably amount of time. A couple of amp-hours for 4+ hours should be a workable starting point.
  4. Small enough to be portable for day hikes. I'd like to keep the size and weight as low as possible for multi-day backpacking in rugged terrain for more difficult to reach SOTA activations.
  5. Price should be as low as possible, utilizing sampled parts when available. If I make this available as a full kit, the price should be under $100 for 40/30/20m.

I started with a Google for some mixers and stumbled on an interesting NXP part. Since I was already requesting samples I grabbed a couple more parts (NXP only allows 3 parts at a time, but multiple counts of varying maximums).

The SA612A is a low-power VHF monolithic double-balanced mixer with on-board oscillator and voltage regulator. It is intended for low cost, low-power communication systems with signal frequencies to 500 MHz and local oscillator frequencies as high as 200 MHz. The mixer is a ‘Gilbert cell’ multiplier configuration that provides gain of 14 dB or more at 45 MHz.

This is a Gilbert cell double balanced mixer that operates up to 500 MHz in a very manageable SO8 package. It's got a built-in oscillator and is available for $2.36 in single quantities from Digi-Key. I was able to sample 20 chips which is probably a lifetime buy if these work out.

The MMG3H21NT1 is a general purpose amplifier that is internally input
matched and internally output matched. It is designed for a broad range of
Class A, small--signal, high linearity, general purpose applications. It is
suitable for applications with frequencies from 0 to 6000 MHz such as
cellular, PCS, BWA, WLL, PHS, CATV, VHF, UHF, UMTS and general
small--signal RF.

This is an RF amplifier that'll go DC to 6 GHz. Small signal gain is on the order of 19 dB. It's 50Ω matched in a SOT package and is available in single quantities from Digi-Key for $2.57. I figure this will make a good RF pre-amp if it's needed given the 14 dB gain from the mixer and will also work as a pre-amp for the transmitter's power amp. I got 5 of them sampled.

The NX3DV2567 is a four-pole double-throw analog switch (4PDT) optimized for
switching WLAN-SIM supply, data and control signals. It has one digital select input (S)
and four switches each with two independent input/outputs (nY0 and nY1) and a common
input/output (nZ). Schmitt trigger action at S makes the circuit tolerant to slower input rise
and fall times across the entire VCC range from 1.4 V to 4.3 V.

Finally, an analog switch that's 4PDT. To save power and cost I'm going to avoid relays and this seemed to fit the bill. I'm not sure if I'll need 4 poles since I haven't sketched out the transceiver yet but it's the highest pole count they had available for sampling. I got 7 of them and it's available at Digi-Key for $0.62 in single quantity. It's a 16 pin QFN package which should be manageable by hand or certainly by hand placed reflow.

Stay tuned for a post with an initial design.

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