23cm NBFM transceiver

IMG_3527This transceiver is published in the december 2015 edition  of Electron, the monthly magazine of the Veron (in Dutch) and in the july 2017 edition of Funk Amateur (in German).

The 23cm band is a beautiful band for all kind of modes, wideband and narrowband: FM-ATV, wideband (stereo) FM, SSB and of course narrowband FM for phone. The 23cm band is very wide so lots of room for experiments: from 1240 to 1300 MHz. Also, quite some repeaters are available, in the Netherlands alone already some 10 repeaters, often located at very high locations of up to 150m above ground.

The only drawback of getting QRV on 23cm is that only a few commercial transceivers are on the market. Often one uses transvertors form 2m to get QRV which work fine for SSB in the SSB-segment (1296-1297MHz), but for repeater usage these are not appropriate because of the large shift. Therefore I decided to design a complete and easy to build FM transceiver.

On these pages this 23cm narrow-band FM phone transceiver is introduced. The transceiver is available as a complete kit, including tin shielding box, a professional printed circuit board and all the parts. You only need a housing, a microphone, a speaker and of course an antenna to get QRV on this beautiful band. This is how my prototype looks like:

IMG_3898Shown is the operational transceiver while receiving our local repeater PI6HVS at 1298.375 MHz, stored in memory M1. The antenna is just a piece of wire, barely visible. The S-meter is shown in the second line of the LCD.

Features:

  • covering the whole band
  • for simplex and repeater usage
  • output up to 1 watt
  • NF 3dB
  • LCD with frequency, shift, RX/TX, S-Meter
  • VFO and 10 memories, all with own shift and CTCSS
  • shift presettable, ie. -28 MHz or +20 MHz, CTCSS 67.5 – 150 Hz
  • just connect a microphone, speaker and an antenna to get QRV on 23cm
  • Easy to build and expandable, software open-source

Schematic diagram and description

The block diagram of the transceiver is straightforward:Dia1

In the final schematic diagram you can recognize the building blocks:

23nbfmsch32

The vco is an ALPS 1679C (IC7) or Y17C which oscillates from 1100 to 1350 MHz. Thesignal is frequency/phase locked in an ADF4113HV (IC8). The HV extension  means that it can handle a tuning voltage of up to 16V (normally 5V, so an additional level shifting op-amp would be needed). The output of the vco is 5dBm, which is attenuated 10dB for proper level at the rx-mixer IC2. The modulation is superimposed on the tuning voltage. To get proper modulation care must be taken in the design of the loopfilter around R13, C39, R14 and C36.

The received antenna signal is passed via a rx/tx pin switch to IC1 which amplifies up to 25dB with a noise figure of about 3dB. After a three pole bandpass filter the rx signal is fed to the mixer IC2, an IAM81008 which has a conversion gain of 8.5dB.  The first IF is 69.3 MHz, which is fed through a bandpass filter consisting of L1 and L2 and accompanying C’s. The output impedance of the mixer is 50 ohm, and so the filter input is matched to 50 ohm by a capacitive tap. The input impedance of the IF chip is 680 ohm/7pF, so here the tap is done at a higher impedance. The IC chip MC3362 contains a dual conversion FM receiver. The first IF here is 10.7 MHz, which is realized by mixing the 69.3 MHz input with a standard 80MHz (computer) crystal oscillator. The second mixer with 10.245 MHz results in 455 kHz where demodulation takes place. Muting is done via T2 by the microcontroller IC11, which shorts the audio when muted.

When transmitting, the vco oscillates directly on the tx frequency. The output is coupled via a -20dB coupler to IC12 which amplifies 25dB, followed by IC13 (+12dB) and T7 (+8dB) to give up to 1W output. After the PIN-diode antenna switch about 800mW remains.

The microcontroller is an Atmel ATMEGA328, which handles everything in the transceiver. It reads the rotary ecncoder and switches, updates the LCD display,  takes care of rx/tx switching, squelch/mute and generating the CTCSS tones. The RSSI signal strength is read from the IF and converted by the internal ADC. As a result the S-meter in the LCD is displayed. The software allows different settings via the menu, stores these in the internal eeprom so it remembers the last frequency when switched off.

The transceiver allows VFO of 10 memory channels, each programmable with their own repeater shift and/or CTCSS. It is possible to continuously scan memories, and the scanning stops for a few seconds when a signal is received.

PCB

I designed a double sided PCB which is professionally manufactured for this project. On the bottomlayer are the SMD parts mounted (blue), while all “normal” parts are soldered on the toplayer.

23cmnbfmassThe VCO can be an ALPS 1679C or a Y17C which have a different position of the marking dot. The output-pin (6 resp. 1) connects to the stripline to R4/R5.

 

User experiences and improvements

The first version of the transceiver is build by about 50 amateurs, and I got nice feedback and also very useful suggestions for improvements. These are all implemented in the latest version 3.1:

  • Software squelch, adjustable via the menu. Two reasons for this change: the IF chip MC3362 doe not support both a good working squelch and a relying S-meter. And the other reason is that the relating circuit in the chip is easily destroyed by shorting the squelch pot to ground. So now the RSSI output of the IF is read into the ADC of the Atmel, and used for muting the audio. Also, the converted RSSI-signal will be translated to a PWM-output on the controller, so that a good working analog S-meter from 0-5V is available for those who prefer this.
  • The decoupling of the first 23cm stripline in the filter with C2 is improved by adding an smd C of 1n in parallel to C2.
  • The sensitivity of the IF is improved by adding a 1k resistor between pin 21 and 22 of the mc3362.
  • C10 and C11 are changed to 27+27p in stead of 27+100pF to match the impedance of the LC filter.
  • Audio noise when receiving a strong signal can be reduced by increasing C24 to 33n.
  • With an extra small C from drain to source on the CLY5 the output can be increased by 3dB up to 1W. I tune the output with a small piece of copper foil, shifted along the output stripline to find the optimal position.
  • to reduce VCO noise the 9V supply is filtered by an additional emitter follower / lowpass.

Measurements

The bandpass filter on 23cm covers nicely the whole band as seen in the plot below. (thanks Ulli from Eisch-Kafka).

23cm_filter_PE1

I also wobbled the bandpass filter at the first IF of  69.3 MHz (1 MHz and 10dB/div).

Naamloos

 

A few pictures of transceivers build by various amateurs:

 

P1030651 P1030648
Wil, PA0WBS
1b3663b8666866909761b7cc2142fd05 IMG_2406
Francesco, IK8PAE Rolf, PE1PTP
IMG_0184 23cm zender
Jan, PA2JS Erik, PE1RQF

5 Responses to 23cm NBFM transceiver

  1. pe1jpd says:

    A new version of the software is out. If you can program the atmel yourself, let me know and I’ll send you the code (source or hex). A fresh programmed atmel is 10 euro incl. shipping.
    73 Bas PE1JPD

  2. Stan PA8C says:

    Indien men in de omgeving een GSM/UMTS zender heeft dan kan de S-meter al een heel hoog signaal aangeven. Er is mij bekend dat er dan een z.g. zuigkring vorr 800-900MHz
    kan worden aangebracht. Wat zijn hiervoor de details?

    73, Stan/PA8C

    • pe1jpd says:

      Hi Stan,
      Inderaad helpt een zuigkring goed om storing van GSM/UMTS ed. te elimineren. Deze zuigkring bestaat uit een (liefst SMD) trimmer 5pF in serie met een stukje draad van ca 1,5cm vanaf de ingang van de rx-mar naar de zijkant van het blik (aarde). Afregelen door met antenne maar zonder 23cm rx signaal met de trimmer de s-meter naar minimum te draaien. 73 Bas PE1JPD

  3. Rolf says:

    Hallo Bas,

    Ik zie dat er een atmega328 wordt gebruikt, heb je die van een Arduino bootloader voorzien en is de software ergens online te vinden zodat we daar mee kunnen spelen?

    Alvast bedankt,
    Rolf

    • pe1jpd says:

      Hi Rolf,
      Nee, geen Arduino bootloader. Ik gebruik AVR-Studio voor de code (in C) en een zelfgebouwd programmertje en PonyProg voor het programmeren.
      73 Bas PE1JPD

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