Sunday, September 13, 2009

HB-1A qrp transceiver

Recently, well, several weeks ago, I came across a neat little radio from China, the HB-1A qrp transceiver. Tempted to order one. Up to now, I could resist.

Searching the internet for more information, I found a schematics diagram (dated 8-Aug-2009). From available information and some guesswork, that's what my understanding of the rig is.

Digital Part
The trx covers 5-16MHz using a DDS (AD9834) controlled by a 16F73. There is a keyer in a 12F629 and a MAX522 dac for generating voltage for the receiver's variable filters (more on that later). Finally, a serial eeprom is wired up to the main controller, which also runs the LCD.

Nothing worth to be mentioned here, the keying looks a little unusual to me... There is one thing interesting however, no mixing is done in the transmitter, although the receiver is a superhet. It appears that the frequency generated by the DDS for transmit is the in band QRG.

Pretty straight forward NE602 / LM386 design. Muting is done by shorting out an attenuated (by a 2N7000) signal by means of a 2N3904. Here comes the first of the two variable filters, a tank-filter design using varactors driven by one of the MAX522's ports. NE602 front-end/mixer wired op to the DDS as LO. IF filtering is done by a variable bandwidth Cohn filter using varactors driven by the other port of the MAX522. A second NE602 used as BFO and product detector in the commonly known manner. The use of the LM386 is slightly unusual to me again, in particular the AF gain, or better volume control, which is done behind all amplification, with a potentiometer just before the phones jack.
No frequencies are given for the crystals, thus, my guesswork with some rough calculations came up with a likely IF of about 4MHz.
Thus, 4,1952MHz it is....

I am not quite sure if I understand the intention behind the relay placed in the low pass. If the transmitter would require an extra low pass filter, this could have been done directly behind the PA, overcoming the relay and in particular the noise it causes... clack clack de-clack...

Personally, I like the concept of the inclusion of the most important SW-BC bands. Seems like a nice companion for vacations, playing radio and receiving the daily news. Looking at the bands the transceiver covers (40/30/20), there will be propagation most of the time.

I was reading on an American page, that development seems to be going on for a version being capable of CW, SSB and DATA. Now, that sounds interesting!
Thus, I will wait for the time being, when ordering now, it wont be here for my next trip anyway.
But again, it's not really expensive... and why not... yeah well... hummm... we'll see about that ;-)