What makes a good receiver? And what about the uBITx

Somebody on the list asked how the µBITx  receiver performs compared to other transceivers.  Allison KB1GMX provides an extensive commentary on  factors that relate to receiver performance with the µBITx as a reference point that will be of interest to constructors.

She notes that receiver performance has many dimensions:

  • Sensitivity
  • Bandwidth
  • Overload performance and dynamic range.
  • Stability, Especially important in the days of VFOs.
  • Spurious signals, birdies and unexpected signals.

The µBITx is unique in a new generation of simple radios with microcomputers to provide the basis for the user interface.

From Allison’s perspective, and work she has done,  the BITx40 had too much gain and required an attenuator most nights. However, this was an easy fix.

The  µBITx when measured on an RX only test bed from about 3-4 years ago reflects a good receiver in the 1-10 mhz range.  However, Allison prefers a bit more RF gain for 10 through 30MHz. As you go up in frequency there is more weak signal propagation and reduced manmade noises and sensitivity approaches the background galactic noise floor. At 40m a 1uV senstivity is plenty, at 28mhz .2uV is more useful.

For selectivity, Allison prefers a tight filter:  2.1 to 2.4khz is fine as SSB is about that wide on transmit  (except for the ESSB people where a 3khz filter would be better).   However, she prefers steeper skirts and that requires more crystals, with 5 crystals being the bare minimum and 7 approaching very good.

Why is this important? Strong signals down the skirts [edges of the filter] are audible if not suppressed adequately and if the same filter is used for TX it assures the unwanted sideband is suppressed.

Overload is a big area for BITx40 users and the same forBITx20 users.  There is  a lot of RF gain and HF bands are known for big signals. Attenuation or circuit changes help greatly.  The uBITX runs without an RF gain control and was optimised for a decently high overload point. So fewer people complain of overload but AGC is a common wish list item.

Stability has been mostly solved by going with Si5351 and Si570 digitally controlled local oscillators in modern HF receiver designs. This requires adding an Arduino microprocessor, LCD display and a some form of encoder to tune. The other implication is there will be signals generated by the microprocessor and in communicating with external devices like the display. It does open up a whole new arena of user interface that didn’t exist in earlier analog designs. An example of this is KB1OIQ’s version of uBitx that is blind user friendly (speech synth output and keypad controls) as its really well thought out.

Adding a Raspberry Pi or similar [STMFxx series] to do signal processing is on face a good idea, but the cost is considerable in terms of software development, and the need for a more sophisticated user interface and power. Power utilisation is an important aspect of a compact portable radio.  Many wish to use batteries and a Raspberry Pi eats  about 3-4watts continuously. Adding a touch screen adds an additional 3-5 watts to that figure. At some point its no longer a simple radio, no longer inexpensive and has become battery unfriendly. Some problems are easily solved without resorting to a computer.

Look at what is being done for the various SDR radios. If you are going digital its smarter to start with a new architecture and build in the computer rather than hang it on like a laptop on the side.

With all that said, yes the µBITx is a decent receiver in Allison’s view. Can it be improved? Yes. However, improvements depending on the use case.  Different constructors will have a different idea of what that may be.


Using a 1N4004 or similar as a varicap or pin diode for AGC control

Allison KB1GMX finds it  odd that every one seems to be bent on levelling the audio volume in the audio circuit.

The Bitx or uBitx has enough gain and handy places that RF gain control based on audio detection works very well. The easy way is replace R13 (ubitx) with a diode such as 1n400x (x=1 to 7) and controlling the  current through the diode to make it behave as a variable resistance at RF.

The current would be about 4-6ma at max gain and decrease to zero (0) at minimum gain.  For that design the AGC range is about 26 to 32db depending on the band. If you feel that is not enough AGC range then add the same mod at R33 and with both the AGC range is near 60+ DB, generally enough.

AGC in this form is less prone to overload distortion as you are lowering gain. The control could be a pot between 8V (or RX-V) and ground and a series 1K resistor to the diode (x2 if using both diodes). That gives a manual gain control. To make it automatic use a circuit to detect the voltage at the top of the audio gain pot and feed that voltage to the gain control diodes. The circuit should be arranged to put 4-8V out at NO Audio and decrease to zero volts with increasing audio.

The 1n400x series with minor reservations makes a fine substitute for a PIN diode, the preferred but more costly device for this function. Beside being widely available and cheap  makes it useful.  It also makes a good 20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 that Allison built over a decade ago to test AGC.  It has been used on several older Tentec radios and more than few of her own design. That said its far from a new idea or design as its documented in EMRFD and an older book (Solid State Design, ARRL press, now out of print).

Jerry KE7ER climbs into the conversation saying:

“I have no idea what the capacitance of a slightly forward biased 1n400x is,
figure 6 here suggests it’s north of 30pf:    Therefore, it might be marginal at 45mhz, and can vary wildly with diode type and brand.

Consensus seems to be that a 1n4007 is preferred over other 1n400x flavors for use as a PIN.   Some experimentation may be required using diodes from different manufacturers:

If you are paying $5 postage to ship in some 1n4007’s, you might consider
also getting some BAP64-02’s at $0.43 each single unit pricing, Mouser 771-BAP64-02-T/R.  These are fully specified for use as an RF PIN diode.