ATS-Mini V/U

[G8PTN]

For general interest, I recently noticed a cheap transceiver chip BK4802P (SOP16 package) which may be interesting for an ATS-Mini V/U type receiver.

It has fixed band ranges and only natively supports FM.

The device needs minimal external components and has a standard I2C control interface. It would probably fit in place of the current SI4732 device in the ATS-Mini.

There is a low-cost board (less than 4 GBP) on the market place sites that uses the BK4802P, which has fixed channel mappings using an EEPROM, MCU and LCD display.

I modified the board to gain access to the I2C connections for the BK4802P, isolating it from the on-board MCU. A quick test configuration of the device using an ESP32-C3 seemed to receive quite well on the 433MHz 70cm Ham band.

Since it is a transceiver chip, there may be the possibility to use the TX side as the LO for a mixer stage, that could be linked with the SI4732 to allow VHF/UHF band AM/SSB. However, realising such a solution in the same form factor as the ATS-Mini may be a little difficult.

A simplistic block diagram is shown below, essentially the down converter is similar to the external devices than have a fixed LO of 100MHz/110MHz. To receive AM Airband, an high side LO would need to be used in this case. Additional filtering stages may be necessary (not shown).

73, Dave

[G8PTN]

Hi,
For interest, a brief update. I added an expansion socket (3x2 female header) to a ATS-Mini V3(B) receiver, providing power (VCC_BAT, GND) and I2C (SCL, SDA) signals to interface with a modified BK4802P VHF/UHF module. The test setup has a separate external speaker, although I have added a pin on the expansion connector for Audio-In, this is not connected it at the moment.

BK4802P module attached to the rear cover via removable sticky strips:

Modified BK4802P module (removed all unused components an cut down the PCB):

For experimentation purposes, the test code provides basic control of the BK4802P chip. Rather than removing the SI4732 functionality, everything is in place, which allows the ATS-Mini to operate in normal mode (HF + VHF broadcast) and VHF/UHF (FM) simultaneously.

The BK4802P seems to receive quite well on VHF and UHF, but unfortunately when using a whip antenna on the module itself, there is RFI from the display and I2C interfaces. If the receiver module is connected to an outdoor antenna, there is no noticeable interference.

The BK4802P has a relatively small (34 locations) register map and most of them can be set to default values. One observation is that the squelch control is rather strange and a little over complicated, some of the thresholds are set via external resistor values and others via internal registers. The increased current consumption for the BK4802P module is approximately 20mA.

It is a rather ad hoc setup, but demonstrates that it should be possible to implement a VHF/UHF (FM) receiver in the same form factor as the ATS-Mini.

I have not yet tried using the BK4802P as a signal source to drive a down converter mixer, which would theoretically allow AM/SSB receive on the VHF/UHF bands, with the SI4732 as the IF-RX. This setup would need more circuitry and RF-filtering.

73, Dave 

A few images are shown below.

SI4732 Default:

BK4802P VHF/UHF:

[G8PTN]

Hi,
A further note on the BK4802P. The chip has a fixed intermediate frequency (IF) of 137kHz and appears to use low-side injection. This means that input frequencies that are 2 * f(IF) lower than the desired frequency will also be received and any use case should take this into consideration.

Examples below.

73, Dave