Personal Page of DM3MAT

Portable QRP Multi-band CW TRX

  1. Features
  2. Circuit description
  3. Sources and CAD files
  4. Some boards for free
  5. Changes in revision 2
Finished CW QRP TRX inside the small Fischer chassis.
Assembled CW QRP TRX inside the small Fischer chassis.

This page describes the second revision of the CW TRX. The page documenting the first revision is still available.

Here, I like to present my small portable QRP CW TRX, that I frequently use at home as well as outdoors, e.g. on bicycle tours. Hence the design constraints were: small size and light weight, low power consumption, CW QRP rig and at least 3 bands. For example, a night band (e.g., 40m) a day band (e.g., 20m) and a contest free band (e.g., 30m).

To this end, one of the mountain-topper rigs by LNR precision would be an ideal choice. However, in my opinion they are prohibitively expensive ($250 +). A much cheaper alternative would be a QCX from QPR Labs ($50). This transceiver, however, is only a single-band rig. Hence, I thought about designing a multi-band version of QCX, as it will not only match all my requirements, but also entertains me more than just building a kit.

Features

Finally I got a portable QRP CW rig with the following features:

Circuit description

A pure CW rig allows to build very efficient PA stages, that can be driven by a simple and cheap TTL circuit. That is, a class C (or sometimes also called a class D) PA. Moreover, the efficiency of these hard switching PAs imply, that there is no need for cooling of the PA transistors at output powers between 5-10W. The transmitter design was therefore complete: A SI5351 chip generates the RF buffered by a TTL driver (e.g., a 74HCT00). The PA will consists of several BS170 MosFETs. Keying and key-shaping will be provided by a simple BD140 PNP transistor, in-line with the PA output transformer. The rich harmonics of the PA will be reduced by a three-stage low-pass filter.

Circuit of the PA and low-pass filter section.
The PA and low-pass filter section. The PA is a simple class-C amplifier followed by a 7-pol Chebyshev low-pass filter.

To this end, the transmitter design differs only slightly from the design of the QCX: Several low-pass filters switched by relays, in contrast to a single LP of the QCX. A broad-band output transformer at the PA transistors in contrast to the tuned output transformer of the QCX. The latter (tuned) output transformer has the advantage of compensating for some output capacity of the PA MosFETs, increasing the efficiency further (so-called class "E" PA) while restricting the PA to work only on a single band. Using a broad-band transformer, the PA will not achieve this high efficiency but will work on many bands.

Circuit of the receiver section.
The receiver and controller section: a simple direct conversion receiver using a sample-hold mixer and a ATMega328 to control everything.

The receiver design was then basically copied from the QCX as the receiver is almost able to work on all bands. In the QCX receiver design, there is a narrow band-pass filter at the receiver input. This band-pass filter is the only part in the receiver design that limits the QCX to receive on a particular band. In my opinion, this band-pass is not really needed: The major issue of the receiver design of the QCX (using fast CMOS-switches as mixers) is low suppression of harmonically related mixing products. That is, if you tune such an RX on e.g. 7Mhz, you will also receive on 14Mhz, 21Mhz, 28Mhz, ... . Unless you are in a very noisy environment, intermodulation should not be an issue, as these mixers are incredible linear with respect to the RF input signal (i.e., low IM) but incredible non-linear with respect to the LO signal (i.e., low harmonic suppression). The PA low-pass filters, however, remain in the reception path and therefore, suppress the harmonics heavily. Hence, I decided to remove the RX band-pass filter and replace it with broad-band RF transformer. The rest of the receiver design remains similar to the QCX.

The last difference to the QCX is the I/Q frequency generation of the phasing receiver: The QCX uses a clever trick to generate the 90deg phase-shift directly using the SI5351 PLL chip. For my circuit, I resorted to the typical divide-by-four technique using a 74AC74.

Sources

The source code, that is, including the schematics, board layouts and firmware as well as an assembly manual in English as well as in German, can be found on my Github repository.

Some boards for free

As for the previous revision, I ordered 10 boards each for this TRX and I only needed 3. I can therefore give away some boards for free. Just contact me and I will send them to you.

Alternatively you may use my new fancy board swap-meet. There you can request boards and even provide left-over boards if you ordered some boards for manufacturing. The latter is quiet easy: head to JLC, PCB Way or any other prototype manufacturer and upload the Gerber files for the boards.

Changed in revision 2

The major difference between the first and second revision is the mixer. The first revision relied on a 74HC4052 which is easy to obtain and available in a DIP package. The second revision resorts to a FST3253 CMOS switch. The latter is also used in the QCX and provides a much smaller Ron and therefore much less damping on higher frequencies.

The controller part was moved onto the RX board. Therefore, there are only two instead of three boards. Additionally, these boards are now connected via several board-to-board interconnects, thus easing the assembly a lot.

Best and 73,
Hannes, DM3MAT