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TS-870S SDR: a testimonial by IZ0ABD

Below are the notes relating to the installation of the IFace interface inside the Kenwood TS-870S of IZ0ABD Francesco. In general, this work is carried out to be able to extract an IF signal from inside the radio so that it can be used to create an SDR panadapter. In general, the application scheme is shown in the following image.

In this case, however, the installation to extract the IF after the first receiving mixer produced unsatisfactory results as, as stated by the manufacturer, the nominal output frequency varies as the filter settings vary. In practice, by varying the width of the IF filters there is a shift in the IF frequency and this complicates demodulation by SDR (each shift in frequency of the RF signal results in a corresponding variation of the BF one – at least for most of the modulations).

For this reason, another strategy was adopted, thus taking the signal to be sent to the SDR receiver immediately before the first reception mixer. Obviously, in this way, the IF signal is not taken but the RF signal, even if suitably filtered by the radio band filters. Refer to the following images for the details of this operation. The use of the IFace card is always necessary as we cannot connect a low impedance load (50 Ohm) in parallel to the radio circuits, a buffer is required.

The RF signal arriving from the antenna is taken upstream of a low-pass filter: from the following diagram, it is possible to identify the exact point where the input of the IFace board will be connected. This is located downstream of diodes D38, D39, and D41 and upstream of the LPF circuit consisting of components L57, C154, C155, and others.

The task of the IFace buffer card is to deliver the power needed to drive the SDR receiver circuits correctly: in general, the intensities of the signals of interest are small fractions of mW. During transmission, on the other hand, there are powers of the order of W or even more, and this can be a problem for the external receiver, especially now that it is working in isofrequency with the radio. Therefore it is good to use the PTT signal to disable the buffer, a unique feature of the IFace. It is therefore necessary to identify the point inside the radio where to take this signal. Nothing could be easier, the TX8 signal is present on the CN7 connector and this is active only during transmission.

Having to supply power, the IFace must also be adequately powered. From connector CN13 it is possible to obtain the voltage required by pins 14S and GND.

Below are some images useful to identify the points on the RF UNIT where to take the various signals: RF, PTT, and power supply.

This image shows how to connect the RF signal from the RF UNIT to the IFace. A thin (0.14 mm) conductor covered in yellow plastic is used. As highlighted, it is not recommended to use a coaxial cable because it introduces a not negligible capacitance in parallel to the components of the LPF 30 MHz circuit and would modify their performance.

The power cable of the board is shown in red and in the following two images it is highlighted where to draw the necessary energy.

The PTT signal is available on the CN7 connector. You can recognize it because it is the one on the side of the IF filter for the SSB and comes to it via an orange conductor. What we have to do is bring this signal to the IFace and it has been done by means of a thin conductor (also in this case yellow in color) placed in parallel directly in the connector (the terminal has been removed from its housing, the thin metal wire wound on it and then reinserted).

At this point, the PTT is connected to the corresponding input on the IFace. Pay attention to which of the three pins is used, the central one is relative to the high active PTT, the left one (in the photo) is relative to the low active PTT, the third is the return to the ground (GND).

In the photographs in the following image you can also see where the IFace board is fixed (on the metal covering the RF UNIT – double-sided adhesive was used) and the SMA-f connector for output of the RF signal. To this comes the output signal through a thin coaxial cable terminated, on the opposite side, with a U.FL connector. The specific PCB connector has been soldered to the IFace (both are supplied in the corresponding kit if purchased).

Finally, this image shows that when you press the PTT on the microphone, the corresponding LED on the IFace lights up (it is the one in red).

It should be noted that this installation involves the isofrequency use of the external SDR receiver. This means that we will have our software set up to receive exactly what the radio is receiving, whereas if we made it work on the IF we would have different settings and also a different functioning. In fact, it has been verified that the IFace buffer correctly disables the flow of energy during transmission, but despite this, the external SDR receiver still picks up a good amount of RF signal (not from the IFace but directly from the environment in which it is located) and must force adjust its gain control circuits by reducing it by a lot. This, returning to reception, has repercussions in a short period (about 1s) in which the SDR receiver is desensitized, then the AGC does its duty and everything returns to normal. Working in IF all this does not happen because the transmission frequency of the radio is different from that of reception of our SDR.

Below are a couple of screenshots related to using HDSDR and an RSP1 to realize the panadapter function together with the TS-870S.

The image above highlights the settings to be used. You can see the use of decimation (very important) and that the SDR receiver works exactly in isofrequency, not on IF.

The image above shows, for those who are curious, which band is visible on the panadapter with the settings shown above.

To purchase an IFace you can use the following buttons.

ATTENTION: Although the installation of IFace 2 is not difficult, it is done at your own risk. TSP S.r.l. is not responsible for any damage, unwanted side effects, or anything else.