7 - Notes about inductors

I have a bit of a mental block about magnetic circuits, I found the site below very useful;
http://www.agder.net/la8ak/12345/n12.htm These are the notes of LA7MI, Stein Torp, translated by LA8AK (sadly sk).

Ferrite Losses;

Apply a highish power RF source to the ferrite core with, for example two turns, record the voltage across the core and increase the level until you get a 20 degree C temperature rise. This can then be stated as the core can take x Volts/turn, e.g 4 V/turn down to 1v/turn (e.g FB43-2401 on 80m)

For cores carrying DC: measuring DC saturation;

Apply a variable DC voltage through a small series resistor (to allow current calculation), and maybe an RFC to one side, Use an inductance meter on the other side, note DC current to reduce inductance by 10%

Testing Broadband Trafo by VSWR:

Load one side with a load and feed the other side thru a return bridge (See Solid Satte Design P154, fig 36)


Working out what XL you need:

working backwards from a assumed working PA you must have a load resistance of (Vce-Vsat)^2 / 2 * POUT, (or else you wouldn;t get the output power).

So, for example 8W and Vce-Vsat=10V implies Rload=6.25 Ohms.,

if two transisotrs are in pushpull the load is two times.  16W RF from two transistors on 12 volt supply must see 12.5 Ohms between the collectors, hence the turns ratio is 2:1 (voltage ratio i.e 4:1 impedance ratio). If the secondary is 50 Ohms then the inductance of the secondary should be >200 Ohms at the lowest frequency. (10uH for down to 3.5MHz or 20uH to 1.57MHz)

Note also that 50 Ohms receiving 16W must have 28.3Volts across it.
If we are restticted to 4 volts per turn then we must not exceed 7 turns.
If we use 2 cores then fewer turns gives the same inductance,

6 - Microphone ampliers - discrete

In keeping with not using any special parts, I would like to make my microphone amplifiers without using ICs (a bit daft I suppose since we wil always be able to get/buy opamps...)

A quick google on "mic amplifiers discrete" found the schematic below, I am unsure who to attribute it to as it pops up in 3 or 4 places on the web, google first found it at

http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm

And Andy Collinson says; amplifier for dynamic mic of 200 to 600 Ohms, gain of +39dB or x90. Q1 is run at low Ic to give better noise response, <5uV up to 5kHz.

I need to fiddle with the values to reduce the bandwidth to 5kHz, as it stands below it is flat to over 100kHz.

Andy also has an electret mic version by omitting the first stage above. Note if using a 2 terminal electret simply connect the series input capacitor to the bottom of the 1k bias resistor.
see http://www.zen22142.zen.co.uk/Circuits/Audio/ecmmic.html

Gain of this is 20dB to way over 100kHz, output Z very low.
I may run this circuit thru LTSPice to optimise frequency response and to test with feeding a 50 Ohm output load...

5 - Two tone Oscillators

These are needed for several things; the standard way of testing linearity in amplifiers is to apply two tone. A series of mixed tones such as 2F1-F2 or 2F1-F2 are created at so many dB below the tones.

Observing these on a Spectrum analyser allows a quantitative measure of goodness - the IMD performance.

An ordinary oscilloscope can be interpreted qualitatively with skill.

I hope to use a SDR to observe the spectrum

The site http://www.w0qe.com/Technical_Topics/imd_testing_of_amplifiers.html
Explains how to measure IMD in transmitters (or linears).

You can use two audio tones injected into the mic socket of the complete transmitter or two RF tones from two oscillators. It is important to keep them well isolated - usually by having a 6dB attenuator on each and then combine them with a combiner such as a simple return loss bridge. The output of this can drive a low pass filter (since harmonics from the oscillator would cause inaccuracy).

If using audio tones, e.g 700 and 1900 hertz - not harmonically related. If RF then they can be very close one application note says 1kHz, another says 1.2 KHz typical for amateur radio use or says use 2kHz or 10kHz - whatever is easy to detect.

Two tone testing is also used to test receiver input circuitry - and there a spacing of 20KHz is often used. Such IMD is related to how a reciever performs when receiving a weak signal when a strong signal is nearby - it would better to use 5kHz since that is a better real life test, however the advantage of the 20kHz separation is that you can compare your design with figures published for many commercial transceivers.
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I will create two XTAL oscillators probably using identical XTALs, one with  a series inductance and one with a series trimmer capacitance - to pull lower and pull higher the output frequencies.

P53 of Eamon's book give a suitable circuit (figure 38)


Each oscillator has a MPSH10 followed by a 2n5109 (with heatsink) into a 7 pole filter (3 x T50-6 inductors and fixed caps into a 4dB pad and a return loss bridge acting as a combiner  - it has a FT37-43) final output is +10dBm per tone. Might be worth adding better voltage regulators, one per oscillator. Also feedthru caps, or at least carefully soldered coax between VERY well shielded sections.

Alternatively the site http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/fa/2003/page1141/index.html gives circuits and useful text - in Dutch it translates ok in Chrome

more later when I actually build something!

4 - Signal Generator - for sensitivity testing

Sources for these are given below, 

Ashar Farhan's signal generator (DXZONE) This is a single output VFO.
His copyright allows free reproduction rights if you keep all his (interesting) text as well, in the interests of space I just give a link here and some notes. He provided two ranges of 3-10MHz and 10-30MHz.with two Hartly oscillators are used, one for each range.  Simple JFET fed from a tapped coil and variable cap, he uses a pull down 220k and 1N4148 on the gate. Coils are 60t(30t) of 28swg would on thick McDonald's(Subway!) straws tapped at 15(8) and 365pF, uses a 2.2pF(4.4pF) coupling cap between the tank and the gate. VFO's feed a JFET, a 6dB attenuator and then 2x BJT(one to frequency counter one to output) The RF output has a further 6dB pad to give a 50 Ohm output. drift 10Hz@3.5MHz, few hundred a minute on 14MHz. He unplugs the digital freq meter when making sensitive readings.

ZL2PD HF RF Signal Generator A simple 450k - 60MHz in 3 switched rangesNice simple circuit - Franklin, tank straight into a Jfet, emitter coupled to another which has a grounded gate, its drain feeds back via a 5p6 cap. o/p across (common) source 330 resistor thru a 120p to a cascode bjt. minimal component count. His tank uses a 10-160pF air spaced and three coils.

ZL2PD SingleSpanOsc 400kHz to 30MHzoutput is 300-400mV into 50 Ohm 

 

3 - Existing test gear and components

I have access to equipment from work, but will try to only use a subset of my own;

Test Gear I currently have;
USB 100MHz oscilloscope Hantek DS2250. (under £150)
Digital capacitance meter (easy and cheap to make a PIC based one - £25)
Digital voltmeters
Power supplies - various
Frequency meter (easy and cheap to make a PIC based one)
I have (just) bought a HF SDR - £90 but I am hoping to be able to use it for 80% of what a spectrum analyser can do.

(as an aside, I brought an old spectrum analyser from work home - about 20Kilos in weight, carried it up the stairs to my workshop/shack/man-den and discovered it didn't work. <sigh>)

I also have an antenna analyer - rigexpert AA-30 which can measure R,C,L
I have several SWR meters and dummy loads as well as a couple of amateur radio transceivers (Kenwood TS520 and an Elecraft K2)


Components: you should build up a junk box full of stuff!

I have bought two resistor kits, one leaded and one SMT (0805 size)
I have a selection of capacitors, some old polystyrene, quite a few polycaps, electrolytics and tantalums and a few micas. I also bought 500V mica caps specifically to build a bank of low pass filters for a PA, I will need more.

I have ferrites - 25 of FT37-43, 25 T50-6 and T50-2 (red and yellow)
Lately I bought 10 each yellow and purple capacitance trimmers.

And 10 x 2n5109, 25 x MPSH10 and 10 x J310, 2 x  RD06HHF1 and 4 x RD16HHF1, I have some diodes of various types but bought 10 x1N5711.
I also bought a dozen BNC Sockets and plugs.

As a strategy I will try to avoid using unusual or unmarked parts, it should be possible for any one to follow this blog and duplicate the work. I may build two or three of something to check repeatability.

Also where I use expensive test gear I will try and research how to build cheap gear, again so anyone can repeat the project themselves.

2 - Order of building - test gear

I think at present I need to build the following;

An RF 2 tone oscillator, this should be two separate xtal controlled oscillators, each possibly pulled to be 20kHz apart (maybe a series coil in one and  series cap trimmer in the other as I don't have many Xtals in my junk box). Each of these will be built ugly style using PCB board soldered up as boxes. The two boxes must be very well screened from each other, Each will feed a 6dB attenuator and then into a return loss bridge wired as a combiner into a low pass filter, 3 coils, 4 caps. (see the separate posts)

Maybe a second RF weak signal oscillator to measure sensitivity. (see the separate post)

Actually might divert into making a grid dip meter as this would pay dividends - with jigs to measure C and L. I could build in a PIC based digital frequency meter into it as well as an analogue meter.

A switchable step attenuator. 0-70dB, low power

A return loss bridge - I'll just use this with my scope and see how I get on. I wonder how I could do phase as well as return voltage across 50 Ohm. (maybe make a more sophisticated bridge with hi-q air capacitors - assuming a two channel scope can detect zero phase (add/subtract scaled voltages - minimum if in phase)

A "sampler" to allow feeding from a 10W or 100W Linear into the SDR(acting as a spectrum analyser).

Dummy loads:  I do have ham radio SWR meters and ATUs with dummy loads as well as a Elecraft K" 10W transceiver and a Kenwood TS530 Valve Transmitter, but I will try and test with simpler (cheaper) equipment.

Farnell have 3W 1% metal film and even 30W or 100W metal film 1% (100 Ohms) FEC2396003, FEC2328286(30W/1%/100R@£2) or EC1114422(100W@£5.74)Need a heat sink on the last two, maybe <0.7degreesC/W

These should allow testing the receive path - measuring sensitivity (MDS= Minimum Discernable Signal) and third order intercept/1dB compression/IMD (Intermodulation Distortion). I will also need to measure noise figure but have yet to learn the theory of how to do that! (I have postponed that for now).

When I build the transmit chain I will have a chain of amplifiers/drivers, each of which should be 50 Ohm i/p and o/p; I would think outputs of the 100mW, 1W, 10W and finally 100W should be ok. With a 20W return bridge I can do return loss measurements of the input of the 100W amp. I will use a conventional SWR on the output, at least on the 10W and 20W.

At present I can see how to use a return bridge to measure amplifier inputs - best done at normal running power. I have read they can be used to measure output return loss by injecting signals into outputs whose inputs are grounded. Sounds risky to me, a spurious oscillation destroys the sig gen. And the test is better done at full power output, hence I think outputs should be tested just by measuring their SWR feeding into a dummy load. Measuring "SWR" of a 100mW amp can be done using a bridge - or just drive 50, 60 and 70 Ohm resistors? When I get experimenting I'll see what things look like.

1 - Starting building a homebrew HF transceiver, Linear and associated test gear

Initial thinking about building my own transceiver; actually I need a couple of 12v 100W Linear HF amps but having the transceiver first makes sense. Makes it easier to test the amplifiers, and means my early work is harder to break.

Sources of data;

I have been reading QEX, QST and RADCOM for 10 years, and built up a library of most of the ham radio books from the ARRL and RSGB. But specifically;

Eamon Skelton EI9GQ - I was inpired by his RADCOM articles and bought his book "Building a Transceiver" from the RSGB shop ( www.rsgb.org )

Experimental Methods in RF Design (EMRFD). Vital reading

I will hopefully remember to specifically quote exact references when I get into the details. I intend to use standard discrete components and avoid specialised ICs