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40m DC receiver (2)

After my previous attempt at building a 40m DC receiver failed (I received nothing but noise while at my Kenwood HF receiver, plenty of signals were heard), I made some changes to improve the sensitivity and to add some features that were desperately needed..

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40m DC receiver (1)

The design for this receiver is based on an earlier design by Steve KD1JV. it is build around two analog switches TS5A3157. These switches have on resistance of ca. 5.5 Ohm 10 times smaller as the often used 74HCT4053 switches.

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Gain distribution in DC receivers

For an upcoming 40m (7Mhz) DC receiver that I am working on, I wanted to know the necessary gain. After building the first prototype, the output was kind of quit. So the question came up, how much gain do I really need for a DC receiver? Back of the envelope calculation : to get from 1uV to 1V is a voltage gain 1*10E+6 or 120dB. 1uV corresponds with S3. S1 is equal to 0.2uV. Thus the voltage gain must 5*10E+6 or 134dB. So that’s is theory. Now let’s have a look at some real-world DC receivers.

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Sweep control oscillator

The purpose of this device is to help you sweep an audio or rf filter. It outputs a 0..10V sawtooth that can be used to control a sinus/ function generator or an rf generator. Together with a CRO oscilloscope, a millivolt meter and or power meter this can be used to accurately characterize a filter in the frequency. Call it a poor man the start of a poor mans spectrum analyzer.

Block diagram

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PM5324 oscillator

A recent question in a CircuitsOnline thread (link here), raised my interest in the oscillator section of this RF sweep generator. The oscillator section is around the (ECO) emitter coupled oscillator topology. This topology will oscillate at almost any frequency with almost any LC combination and is not critical with transistor types (of course using LF transistors won’t work at UHF..). The disadvantage is that the current through the common emitters needs to be carefully adjusted. if not the output won’t look like a sine wave output and there will be many harmonics (though this could be useful at sometimes). Also, because the transistors will saturate, the upper frequency is limited.

Emitter-couple- oscillator (ECO)

Not so, the PM5324 oscillator!

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LCR bridge

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Scope probe storage

Usually the workbench is cluttered with oscilloscope probes, serial cables, programming adapters and the like. Before a cable can be used it first needs to be liberated from beneath a pile of projects. While doing so, you accidentally move a wire on your current project which shortens out an essential part of which you do not posses any spares in your junk box and of course it is Saturday 1701h so the shops have just closed for the weekend…. 🙁

Even it doesn’t describe your workbench, this could still be a very usefull project! It pays to be carefull with your oscilloscope probes. The tips are tiny and easy to damage and the coax cable to you scope is tiny and fragile…

VHS-HZ-200 cover
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L-meter

The usage of inductors in RF circuits is often intrinsic to its operation. Unfortunately, the measurement of small values of inductance is often difficult without specialized equipment. If, you have the ability to measure frequency, either directly with a frequency counter or indirect with an oscilloscope, this burden can easily be overcome.

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Walkie talkie (2) transmitter

Below you see the block diagram for the walkie talkie transmitter.

Transmitter block diagram

The carrier oscillator is crystal controlled to guarantee sufficient stability. It is much easier to create a frequency stable carrier by means of a crystal than with an ordinary LC oscillator. The latter would require carefull selection of components and very careful construction to ensure sufficient frequency stability.
The minuscule voice signal from the microphone (a couple of millivolt) is amplified to line level in the audio amplifier. To have sufficient modulation a signal of circa 3- 4 Vtt is required. The bandwidth of the audio amplifier is limited to circa 3.5 kHz.
This is still wide enough for good intelligibility but restricts the used RF bandwidth.
Finally the output is filtered to remove unwanted harmonics.

The oscillator Q1 and modulator Q2.

Walkie-talkie (1)

Lately my 6 year old son has gotten interested in walkie-talkies. Now, I could go to the nearest toy-store and buy something but where is the fun in that? Since, he is getting more interested in technology it would be nice to design something that he can, at least to some extend, help me with assembling.

Features
Easy to build
Low cost
Crystal controlled
Sensitive Receiver

Design goals:

  • Usage of ham bands, so it is legal
  • No spurious emissions
  • Auto power-off (I think you don’t need to have kids to understand why … )
  • Low-power transmitter, just the end of the street
  • Sensitive receiver to match
  • Need to build at least two, so no exotic parts
  • Battery powered and thus low power consumption

The set-up of the transceiver will be quit simple:

The transceiver will by default always be in receive mode. Unless the Press-To-Talk button is pressed during which short time it will transmit. At other moments the transmitter will be switched off to conserve power. (The auto-power off function is not shown in the block diagram).