LOW PASS FILTER BLOCK 3.5-30MHz

This board consist of the following modules:

5 chebyshev type low pass filters to cover all bands

High voltage (210V) scheme

Antenna RX/TX switching

SWR directional coupler and measurement of Vfwd/Vref

Optional LPF (160m) connector

This module ensures clear harmonic output of the linear amplifier, also fits the components for antenna RX/TX switching. Following the “click-less” conception, there are no mechanical relays in the circuit. After long searching for suitable power switching component I found VISHAY high voltage rectifier diode S07J that met all the requirements for reverse voltage capacity, reverse recovery time, forward current and reverse voltage. So S07J were used in the project to ensure good switching of TX power to the antenna via proper LPF and also reliable isolation between each other low pass filters not selected for this band portion.

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High voltage generated from input signal polarizes unused switching diodes in reverse direction ceasing the chance for signal leak. Same votage is used in RX/TX switch for completely disconnecting receiver front end on transmit.

 

 

Block interconnection diagram

My homebrew HF transceiver consist of the following printed circuit boards:

  1. Front Panel assembly – plastic front panel, control knobs, FSTN display and keyboards, control board
  2. Mother board – fits all connectors on the rear panel, power supply, I/O controller (second CPU) intended to be a chassis for all RF boards and options, as long as “cable harness” and board interconnections.
  3. IF/AF Board – IF and audio circuitry, IF AGC 134kHz. Second narrow band crystal filter (option). AF filter (option)
  4. RF mixer board – RX/TX switching, receiver front end, first RX mixer, TX mixer, first roofing crystal filter, ALC circuit. Noise blanker (option)
  5. Band pass filters – Front end filters 160-10m (160m option)
  6. SSB GEN board – forms both CW and SSB signals, microphone amplifier, compressor unit (option)
  7. LO board – local oscillators, side tone and beep tone oscillator. First oscillator and BFO oscillator.
  8. Compressor unit (option)
  9. Automatic antenna tuner (option).
  10. Transverter/RX antenna board (option).
  11. Power amplifier assembly – consist of two boards PA50W and LPF nested directly onto the rear panel

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Crystal filter 9MHz 2.7kHz

One of the most important modules in the home made HF transceiver project is the crystal filter. It is made of individual quartz crystals with matched parameters – Fser. (+/- 15Hz), Lm and Q factor > 100 000.  A large quantity of 9MHz oscillator crystals were purchased to extract only few to fit those requirements. Three type of quartz filters are used in this project – main (roofing) filter, narrow filter for SSB or CW modes and noise filter. All the filters are internally impedance matched to 50 ohm input/output circuitry and individually aligned.

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Here is information about first – wide 2.7kHz SSB filter:

Filter type                          Chebyshev 8 pole

Center frequency             9MHz

Bandwith at -6db             2.7kHz

shape factor -6/-60db    1.75

IN/OUT impedance        50ohm

Band pass ripple              <1db

Stop band attenuation    >80db

VNA plot

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Circuit diagram. Chebyshev 8 pole ladder type crystal filter

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Custom multicolor backlit FSTN LCD for HF QRP transceiver project

In order to fulfill all the requirements to show different modes and parameters we decided to make our own liquid crystal display.

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It is designed and built on the well proved – FSTN technology with high visibility and contrast.

Technical data:
LCD type                   FSTN
Mode                          positive
Poarizer type            transflective
View direction         12h
Connect type            pin
Glass thickness        1.1mm
Working Voltage     3/5V
Multiplex level        32/5
Operating temp      0 – 50C
Storage temp
Content                   Digits/Symbols
Measures                114.3 x 27.94mm
num of symbols    149
backlight type       LED multicolor

It was firstly designed with 5 multiplex lines but soon after receiving first samples it turned out that this is not suitable as there is no driver chips on the market with 5 multiplex lines so we redesign the LCD with 40/4 so the most common Phillips chip can drive it.

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My new HF home made QRP transceiver – first sight

Here it is – our home brew transceiver is alive.

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Die plastic mold is not ready yet so we ordered CNC machined aluminum panel.

All the plastic knobs are 3D printed (waiting for the plastic mold)

FSTN display is super nice -bright and sharp in all viewing angles. We approved the 5 samples sent by the manufacturer and now wating for the first production batch.

Stay connected for future news!

73, LZ2JR/AC9IJ

Weighted knobs MAIN and SUB for Elecraft K3

Elecraft K3 is a super performer HF radio and has been sitting on my bench for many years but the knobs need to be improved. To add some more precise movement and better aesthetic effect we made these heavy bronze knobs MAIN and SUB. When the friction between the knob and  the textile ring is proper adjusted tuning is fast & easy and you will like your equipment even more.

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BITX multiband metal enclosure

I purchased ready made metal enclosure – EM series

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Here is how it must look after making the front panel:

front panel silk screen

front panel silk screen

 

 

 

VFO /variable frequency oscillator/ by sdr-kits.net

Here I will share my experience with PA0KLT VFO kit.

VFO kit based on Silicon Labs digital synthesizer chip Si-570 arrived too fast from UK with ordinary post service /a week or so/. I purchased new version with implemented additional buttons for RIT and FAST functions .. also with ability to exchange the main mechanical tuning knob with optical encoder. Building the unit is not difficult at all and passed straight forward… even placing Si570 chip is not a pain.

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The unit comes to alive from first attempt. I’ve used it for two months now with my multiband BITX – home made transceiver.

I like that it has rock solid frequency stability and accuracy. It has well organized config and memory menu for calibration, writing in memories, assigning IF /intermediate frequency/. Mode selection – CW, USB, LSB and AM. Two VFO’s and memories

What I dislike is:

1. It cannot memorize the last position of the VFO when shutting down… After switching power supply on again it always runs from VFO A?!

2. Band decoding is not correct according the manual. The table in the manual is not correct. Then I decided to contact PA0KLT directly so he did send me correct band decoding tableHere I suggest another table which is more suitable for BITX multiband transceiver.

3. IF shifts for CW/USB/LSB cannot be adjusted separately. For LSB/USB it is assigned to be ±1.5kHz and for CW ±0.75kHz which is not flexible and limits the use of different crystal filters /wide and narrow/ .. also different positions of the carrier crystals. In those cases the frequency shown on LCD would be incorrect!

4. The microcontroller used in the VFO does not support other type encoders with more PPR /more than 25ppr/ … so the tuning speed is way too slow so use of X5 rate or ‘< ‘ ,  ‘>’ buttons are mandatory.

My BITX Multiband transceiver

Recently I decided to join this interesting project and purchased semi KIT from Sunil – VU3SUA. Kit consists of all three boards – exciter, LPF and BPF and parts for the exciter board. Multiband concept of the BITX transceiver has been developed by Andy – G6LBQ. Further I purchased toko inductors from Spectrum Communications – UK and PA0KLT Si-570 VFO kit from sdr-kits.net.

Here is how it looked in the begining:

Some hard to find parts as double balanced mixerADE-1 I supplied from ebay.co.uk and other internet sites.

Si-570 is excellent chip and gives pretty much signal at the output… so it must be conditioned to suit 7dbm level requirement for diode DBM mixer ADE-1. Also suitable low pass filter should be added in order to achieve good sinusoidal signal. As seen on the photos I used an old KVG 2.4kHz crystal filter XF-9B with carrier crystals.

As seen on the photos  I placed some tin shields between the stages of the exciter board and some additional grounding on the bottom side. This and placing low pass filter on BFO output completely cured so called BIRDIES problem. Also this improved carrier attenuation on transmit.

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