There has been a lot of buzz about the BITX transceiver over the internet. A friend of mine expressed his need to get a RX in his room as a bedside Receiver for monitoring DX and other traffic on the 14Mhz Band. Although propogation is bad on 20M we still decided to go hunting for the best Rx design with the usual criteria :
- Should be a low cost design without compromising on performance.
- Should be a easy to construct and troubleshoot design.
- No hard to find / Exotic components.
- Should be well documented as the Rx may be used for Research at a Undergraduate Level
We were still not satisfied after looking at almost all designs on the internet. After a Quick convincing session me and my friend decided to make a Rx based on a exsisting TRX design and we did not go anywhere else except The BITX site.
We decided to make a few modification though. The extra amplifiers needed for Tx are to be omitted and a Xtal Controlled LO with varactor tuning is to replace the LC VFO (We know that the tuning range will be limited but it is a sacrifice we are willing to make to avoid the Coil.)
Other Inclusions are a 3 Khz Active / Passive Audio Filter for SSB reception along with a AGC with a Override Switch. An S-Meter is also a fancy option which we might just add.
The only thing troubling us (lest we forget the College QRL) is the VXO. I was planning to get a standard 4.00Mhz Crystal available in the market for usage with the clock timer chips. The trouble is to pull the Frequency of the LO upto +/- 60 Khz using a super VXO design. I was thinking if these xtals would be able to do that.
The first step in this project would be is to complete the LO along with a Buffer and feed the signal to a frequency counter to see how far these xtals could be pulled.
Rest in the next Installment :)
73's
2 comments:
Several hams have talked about building just the receive side of the BITX20 transceiver. The only thing to change is the input impedance of the receiver amplifiers. Not building the transmit amplifier sections would change the receive amplifier impedances a small amount.
When you build the VXO, you will be building a crystal oscillator with inductive and capacitive elements in series with the crystals. If you add a switch to short out the crystal you will then be able to tune the oscillator as if it were a VFO, which it really is.
If you sample the audio ahead of the gain control you can rectify and amplify this voltage to drive your S-meter.
This receiver has no AGC (Automatic Gain Control) which may be a problem for casual listening. The BITX20 discussion group has talked about several ways to add receive ALC to the design.
The crystal filter of the BITX20 is designed to provide a 2.5 KHz bandwidth. You can change this by changing the capacitors that connect between the crystals to ground (more capacitance equal narrower bandwidth). Try doubling the capacitance values as a first try.
Hope this helps you get started on the project. When finished, please post your results to the BITX20 discussion group so others can admire and benefit from your efforts.
Thanks,
Arv - K7HKL
Thanks Arv!
The amp impedance is not giving problems but there is a slight hum when i tune my bfo. The xtal filter bandwidth is just perfect for me . although my xtal filter uses unmatched ones,my rx is great for Cw and ssb. i have added a small switchable AF filter to enhance the listening capability.
I will post the info soon on the bitx 20 group soon arv.
Thanks!
ragav
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