Multi-tuning AM selector

Question

I'm thinking about the design of an AM receiver frontend that needs to select between three channels. The community suggested that I investigate manually wound loop antennas, and they seem like a nice choice.

AM loop antennas are designed for resonance at a tuned frequency within a range allowed by a variable cap. The loop is inductive, so at the tuned station the loop and the cap resonate.

For my application, a cap with a wide range is not a good idea. I'd prefer to have three individually tunable sections that can retain their tuning, to be switched digitally. A circuit diagram follows (limited by my poor tablet software):

The sections are tunable around 3.33, 7.85 and 14.67 MHz, respectively. In each of the three tuning sections, the first cap is variable and the second one is fixed. The trim caps are 3-10, 4.5-20, and 8-45 pF, respectively. Once the trim caps are tuned they shouldn't have to change (much), barring aging components, mechanical antenna distortion, etc.

The switches are digitally controlled RF switches. An SP3T switch or individual switches are possible.

Is this a reasonable approach? For the RF switches can I use a topology similar to a digital pass gate with parallel NPN and PNP RF transistors?

Answer 1

If you follow the cap switching topology, a PIN diode suitable for use at your frequency of interest (i.e. MAcom MA4PH301) would be less complex than the transistor switch you mentioned in your question. In the sketch, L1 & C1 form the resonant tank circuit. R1 is chosen to limit the forward bias current from your control voltage to a value within the limit of your chosen PIN diode. The PIN diode is acting as a switch, not a variable capacitance device (varactor).

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

AM loop antennas are designed for resonance at a tuned frequency within a range allowed by a variable cap. The loop is inductive, so at the tuned station the loop and the cap resonate.

Regular loop antennas i.e. ferrite rod antennas can be tuned but the tuning is quite broad because they don't have a high Q factor. This means that no-matter how good your capacitor tuning is you might still receive several stations simultaneously and not be able to distinguish between them.

If you re-wound the coil to give a higher Q you could get a lot better selectivity when tuning but, then you hit the next problem - the overall Q of the tuned circuit will be different at one end of the tuning band compared to the other end. I'm not saying it's a showstopper but you should be aware that what might work at one end of the band may not produce the desired results at the other end of the band.

This could be mitigated by also having a different value resistor in parallel with the switched tuning capacitor.

Answer 3

If you want to achieve digitally tuned stations I'd suggest you would be better to use tuning diodes (varactor diodes) and a single tuning inductor.

NXP are my favorite supplier, and they make diodes up to 120 pF or so ….for example the BB201 which can provide a decade of adjustment (10-100 pF) with only 20 V drive.

To sum up the comments: It's your choice ….I simply made the suggestion that tuning diodes could work for you. It's up to you to decide if that does work, and which device is best. The BB201 is the only device I have used, hence suggesting it as an example. The BB171 does 17:1 ...that would seem not to yet fit your 19:1 requirement. Notice however that the minimum reverse bias is always at some level 0.5 or 1 V, so there is room to play at the low voltage end.

There are other devices that meet your requirements, you could have easily looked them up yourself.
Of the several I looked at the Infineon BB640 among the highest lo/hi ratio at 19.5-25:1, but again it's up to you to decide what fits your need.