As written in previous post, the main components on typical PT4302 receiver module includes one PT4302 receiver chip, one crystal oscillator and one 10.7Mhz band filter, we can say all PT4302 is same standard, but we should carefully choose the other two components, as they will affect actual performance / operating distance greatly.
First we will discuss about the value and other parameter of onboard crystal oscillator, and we should refer to PT4302 official datasheet for the details, as follows.
All timing and tuning operations on PT4302 are derived from internal one-pin colpitts reference oscillator / the oscillator, usually we will use onboard crystal oscillator instead of the other option – external timing signal.
When crystal oscillator is used, the minimum oscillation voltage swing is 300 mVpp, for externally applied signal, the source should be AC-coupled and limited to a voltage swing range from 0.3 Vpp to 1.5 Vpp.
As with any superheterodyne receiver, the mixing between the internal LO – local oscillator frequency fLO and the incoming carrier frequency fTX must ideally equal the IF center frequency 10.7Mhz, the following equation maybe used to compute the appropriate fLO for a given fTX.
$$f_{LO} = f_{TX} \pm 10.7$$
Frequencies in above equations are measured in Mhz, please note in above equations either plus or minus, means two different types of mixing, when fLO > fTX, it’s called high-side mixing, which result in an image frequency above the frequency of interest, when fLO < fTX, image frequency is below the frequency of interest, low-side mixing is recommended as it can save power consumption, and now we can use following equation to calculate reference oscillator frequency fREFOSC, which we use to purchase onboard crystal oscillator.
$$f_{REFOSC} = f_{LO} / 32 = ( f_{TX} – 10.7 ) / 32$$
To save calculation time, we have listed calculated value for most common receiver frequency used, table is as follows.
| Transmit Frequency fTX (Mhz) | Reference Oscillator Frequency fREFOSC (Mhz) |
|---|---|
| 315.00 | 9.509375 |
| 340.00 | 10.29063 |
| 390.00 | 11.85313 |
| 433.92 | 13.22563 |
The reference oscillator frequency is close to 10.7 MHz intermediate frequency. It is necessary to avoid signal trace coupling between the reference oscillator and intermediate frequency to prevent degradation of receiver performance.
Second we will talk about choosing correct 10.7Mhz cermic band filter, to choose the right one, we will focus on two parameters, one is 10.7Mhz frequency, the other is 180 KHz BW or bandwidth, if either parameter is wrong, then the combined performance of receiver module would be worse than designed.
The most common two brands or models that we choose are Murata and TDK, for 3-pin dip type, Murata’s package is dark red, while TDK’s package is light blue, and the precise model we will use is SFELA10M7HA00-B0 for Murata, or you may see some marked SFE10.7MS3-A, which is the old part number for same component, while for TDK brand, the correct model would be TDK FFE1070MS10RBL, the two has similar performance, and are interchangable, we usually choose Murata if possible.
Sometimes the wrong IF bandwidth other than 180kHz will also work, as long as your center frequency is correct, but since most RF transmitters have +/- 75kHz frequency deviation range from designed center frequency, so we would highly recommend using 180kHz lead band filter in production receiver board.