Sourcing Parts - How times have changed

No - I'm not getting a kick back from Mouser though a voucher would be welcome.

Well it's certainly a turbulent supply chain at present. I was going to make a throw away comment within my conclusion to 4:1 transformers about sourcing ferrites. However, I realised that since the supply disruption began my perception of where to source parts has changed greatly. If I thought 2 years ago that Mouser would be my preferred source I'd have checked myself into the mental hospital. But for good reasons Mouser presently does justify having a look.

The supply chain issues also impact my plans. In the past I have held PCB's and some parts for those wanting them. But freight to me, and then to you, meant I was moving towards using a PCB assembler and having them send direct to you pcb's populated with surface mount parts. But they can't fit parts that are out of stock, and I can't continually have to make substitutions to overcome this. It is awkward all round.

So let's look at some outlets in the order in which I once used them.

Aliexpress - Avoid

Many reasons: 

1. Fakes. Fakes. Fakes.
   
2. You can't find what you want without wading through pages of rubbish.  I don't know how a search for "ferrite" returns results like diodes, transistors and electrolytic capacitors. 
3. In general it is now expensive
4. No consistency from one order to the next.
5. Chinese sellers rarely meet specifications
6. It was posted so you must have got it scam.
   
7. Disputes rarely amount to a refund.
   

I recommend ignoring Aliexpress at this time. Occasionally I buy a plastic project case or hand tool but that's about it. At the very least they really need to fix their website search engine before  it becomes worthwhile again.

LCSC

Still a reputable source for jellybean parts like chip capacitors and resistors and low cost semiconductors. However, move away from generic and prices have become too expensive. eg the STM8S003 purchased for $0.30 two years ago at one stage was quoted as $3, though the price is easing now. But they rarely have them in stock. And the filters when searching for parts, while adequate, is not as useful as Mouser, Digikey etc.

Mouser

While prices at Mouser have risen, the increase is not as great as say LCSC. Yes, Mouser started from a higher base but today I find:

• many parts outside jellybean status are available at Mouser but not LCSC,
• Mouser pricing is competitive
• Mouser holds many more stock items. eg I couldn't find any ferrite toroids at LCSC but Mouser has  a large range. 
  

 

Arrow, Digikey etc.

I still look at other tier 1 suppliers but I haven't found a compelling reason to choose them over Mouser. Arrow would be my least preferred alternative because their filter function when searching is cumbersome. 

It might simply come down to freight costs where you live.

Freight

This is perhaps the game changer. If you can find it on Aliexpress the cost with freight is too high. The free post bargain we used to enjoy has gone. 

LCSC freight costs have also increased. Must be a great time to be in logistics.

Occasionally you find "freight free" items at Mouser and this really does push Mouser into the lead at this time. Get the order value high enough and freight free becomes a possibility. With prudent purchasing I have not paid much, if anything, for freight for my last few orders. 

Conclusion:

Lead times are a big issue for items out of stock for all outlets. I run a cart at both LCSC and Mouser. Mouser are surprisingly competitive now and have a much wider selection outside the jelly bean range. Freight costs tend to see parts in my LCSC cart getting dropped in favour of sourcing from Mouser. The overall result is that today the ratio of Mouser to LCSC deliveries here is about 2 to 1 in favour of Mouser. That's a large change from  perhaps 3 to 1 in favour of LCSC a few years ago.

Happy to hear where your preferred source is. I'm always looking at alternatives.

 

73's

Richard

4:1 (200:50 ohm) Transformers - Two Suggested Cores

 Still more testing but I have narrowed it down to a choice of two:

Small Binocular Core ~A$0.30 each

My initial test with this binocular core was only 3 bifilar turns due to the size wire being used. I  suggested if 4 turns didn't perform better it should be put to one side. I twisted up some fine wire and had little difficulty passing 7 turns through the core. It appears useful across HF:

 

 

 Small Toroid Core ~A$0.20

Now this core was a real winner. It's like two FT-24-43 cores stacked. I had tried some smaller cores which appeared to be too small for HF. I wound 10 turns and measured in my fixture and thought this could work. I rewound with some twisted wire and increased the winding to 13 turns. It just felt right looking at the curve for 10t.

The result was outstanding and far exceeded my expectation being useful beyond 150MHz as the sweep below from 1MHz to 150MHz shows. 

I am very happy that a small amount of time with the nanaVNA, some systematic investigation and thought have resulted in a 4:1 transformer I can use from 160m to 2m.

73's

Richard

General Purpose Transistors in RF Amplifiers - Ouput Transistor Testing

After having my eyes opened by testing various buffer transistors I began substituting output transistors. Before testing currently available transistors I thought it useful to compare 6 alternatives in my junk box. These included fake 2SC2166 and 2SC1173 transistors, some genuine recovered CB finals, and a 30 year old BD139. All surprised me by appearing to work quite well. However, I wasn't driving them to get 1 Watt at this point.

Some of the testing results surprised me so I decided to give LTSpice a go to see if I could learn why I was surprised. That was a few days in my life I wont get back. However, using LTSpice made me accept that my spreadsheets may be in danger of becoming obsolete. The biggest challenge is being restricted to transistors for which a suitable model exists. But if you can find a spice model, LTSpice allows you to quickly rule out a device once everything is set up.

I have tested a number of devices now and I could not find a device that worked to my satisfaction. Issues noted were:

• the output dropping as the temperature rose, sometimes by 20dB!,
• distortion that required large increases in standing currents, and
  
• a roll-off in gain that ruled out use beyond 7MHz

I found that driving 50 ohms from a 13.8V appears to be pushing the envelope. If I increased the supply voltage, or changed bias resistors to increase the standing current, I could get 1Watt out. This came at the expense of the transistor dissipating 3 watts.

Conclusion:

From my measurements the KSC3503 is worth trying. If you settle for less than 1 watt you will be happy. At 7MHz and below the TTC004B is also worth considering.

However, from results to date, a working output transistor recovered from the junk box is still better than trying to buy a general purpose transistor for RF output stage applications.

Next Steps:

Investigate if parallel transistors will meet the 1 Watt target. (Update - yes they did and very nicely. See posts tagged "Parallel" for details.)

73's

Richard

General Purpose Transistors in RF Amplifiers - Buffer Test Results

Please see the previous posts to understand I am trying to replace obsolete transistors in my standard Class A amplifier chain with something readily available today. Especially for the output stage.

Having settled on a driver transistor I thought that replacing the 2N2219 with a generic buffer transistor was going to be without issue. However, this assumption was wrong and the final stage testing has been delayed.

The 2N2219, used as the buffer, has a claimed transition frequency of at least 250MHz. I've spent the last 40 years playing with small signal NPN transistors and regularly noticed in the datasheets Ft of 300MHz or more. So I assumed that pretty much any transistor I dropped into the buffer stage was going to work.

It wasn't that simple. I started with something marked G1. This was ~1.2dB worse that the 2N2219. Hmmn. 

Grabbed something marked 1GM. This was ~1.5dB better than a 2N2219. Hang on. If this was a MMBTA05LT1, a Motorola NPN part marked 1GM, it should have been comparable with a claimed Ft of 330MHZ at 25mA.

An element of confusion was setting in. I decided to put the junk box recovered parts away and grab some nice new and fresh from packet NPN transistors. 

The first was something labelled a BC549 from a long forgotten Aliexpress vendor. I agree, this could still have been a junk box part. It was marked G1 and was better than the first "G1" I tried and the 2N2219 itself. Rolloff was improved by 1dB over the 2N2219.

After trying many such parts I tried a 2SC3356R, Ft of 7GHz. The rolloff at 20MHZ was now 2dB better than the 2N2219 version. A similar outcome was achieved when I recycled something marked R22 from a Philips FM900 VCO board. Some claim this marking is a 2SC3356. I'm not sure this was a 2SC3356 however it was a high Ft device.

Clearly, the higher the Ft the better. Which I hadn't expected given the feedback networks being used.

I crunched up a spreadsheet with a Hybrid Pi model of a Class A amplifier without collector base feedback to see what difference Ft made. With all assumptions unchanged, shifting Ft from 100Mhz to 330MHz results in a lot more gain at 20MHz. Collector base feedback reduces this gain, but the numbers supported selecting a buffer transistor with a higher Ft.

Later I fired up RFSim99 and loaded up the S parameters for a Renasas 2SC3356. Fiddling with the emitter and feedback resistors suggested a minor variation in values would be better to improve the match to 50 ohms. I will use those values going forward. 

There are many 2SC3356 transistors on LCSC.  Since Renasas has discontinued this part I figured it would be prudent to include some in my next order so I opted for this 2SC3356 since it had curves that resembled those in the Renasas datasheet.

Conclusion:

Recycle, or spend a few extra cents if you're buying a transistor, and get something for the buffer with an Ft greater than 1GHz. This may be counter to the conventional wisdom that HF circuits do not need such a high Ft. But the difference could be as much as 3dB per stage. So for low level amplifiers with feedback to prevent oscillation I argue it makes sense since it could save an entire stage.

Now I can test a few final transistors.

73's

Richard