SPF5189z Low noise preamp 23cm

 14-4-2018 updated


Above the low noise  preamp 50 - 4GHz    ebay  6€

Tested  2 small porto antennas
1 Baofeng UV antenna left   2nd best signal on PI2NOS
2 eBay DIAMONDSRH805S  wide band 144/432/1200  it works for its size as expected
3 piece of wire :   best signal in the setup...

Unit under test at workbench.
It was stable  50mA  at 5V   few days,  but sudden the current went up to 100mA
And i could  no longer adjust it with the negative bias Voltage.
The - Voltage went to zero and the SPF input port became low Z.
This SPF5189z was broken  no oscillations, no input overload just it broke while i was receiving the PI2NOS 430.125 using a small antenna at the workbench.

The specs of  SPF5189z  say it has an internal circuit  regulating the bias current.
Replaced . again   with an other of the 5 spare SPF i have.
The new one again could be adjusted at 50mA  5V 
The current raises to 150mA  when removing the  negative gate voltage.
Where is that internal bias circuit ?
I suspect  eBay SPF5189z devices like al other eBay semiconductors..
But the pre-amp is working good
Reception of the RTL2832u USB receiver  is improved a lot with this preamp

This year tested the shielded version SPF5189 at 2018 Heelweg microwave meeting
result:  0.73dB nf @ 23cm   0.45dB nf @70cm out of the box!

But the un shielded version  pre-amp above drawed right from the start  very high current at 5V
over 200mA  way more then the specified 90mA
Thought my SPF5189z must be wrong so i replaced it
But with the other SPF5189z  same  very high current practical short circuit
it could not run  at 5V
Reduced to 3V  with 3x Si diodes and a 1 OHm serial resistor.
current still about 100mA .  This way it was running months with good result.
Enough through-gain to get over 30m RG58 -6dB at i.f. 146MHz
Daily copy  5 Beacons  GB3MHZ OZ7IGY ON0TB DB0VC DB0XY   PI7ALK
But started some issue  with bad joints in the  SMA to N  connections
Added  SWR bridge to monitor the TX   down in the shack.
Placed a new SPF5189z preamp, a type with metal shield drawing correct 90 -100mA at 5V
Al seemed okay but 23cm had cracking noise in some directions like never ending  thunderstorm QRM  And also the usual S5 noise level at 146MHz  was only S1 in low  QRM directions
QRM at 23cm a new experience could it be nearby high GSM signals 4G
I will have to mak my own QRM hunting device

Agentschap Telecom was here to investigate my reported QRM problems
Still unsolved Solar Edge noise and digi 200kHz apart carriers.

But AT could not see noise raise on their portable spectrum analyser using  special band antenna's

I have S5 noise raise at 50MHz with a 2 ele directed to the Solar panels 50m North of me
the digi trash carriers peak S9 +  at 6m 50.198
Repeating  every 200kHz  in the whole 1MHz to 150MHz band  SolarEdge  works as a broadband QRM transmitter !

The source is in the CPU part of the controller that i found out with the IC706 at 50.198MHz and a short pick up wire
Close to the SOLAR EDGE, the digi carrier trash clearly was strongest near the Solar Edge display not on the PV cables
Those cables need to be twisted i got an PV cable powerline filter from SolarEdge but since the RFI comes from the internal CPU the filters did not help

Digi thrash cariers with  maximum of 9+20 at 14.198 MHz on a dipole   repeated every 200kHz.  up to VHF !

Frustrating that QRM like this is legal and malfunctioning Solar devices can operate like this.
A ham would think just build TX and skip the LP-filters..
My own Solar Frontier SAF3600 system  is absolute RF quiet.

To continue with  the SPF5189z  pre-amp
No signs of self oscillating. Matching to the 23cm slot dipole SWR 1:1.
The antenna relay okay, so what else could it be then the pre-amp became bad?
Still receive Beacons much weaker then usual.
I had 5 spare  SPF5189z   replaced the SPF same  high current at 5V
The Device specification  says  5V 90mA .

Diode testing, the input shows 0.6V  barrier  and reverse 1.4V
The output is just 0 Ohm's  both way's all 5 devices

Then with a second diodetester  reverse on the input with a small negative Voltage
The output Z  went  up from zero to a high value !
As if this SPF5189z would need a negative input bias Voltage.

So i made a smal negative voltage source adjustable and through a small choke coil applied it to the input gate of the SPF5189z and voila  smoothly the Supply current is adjusted to the wanted value.

Regulating -1V  40mA  to  -0.65V is   90mA

next question is the preamp stil working
A simple test showed that it does amplify I can let it self oscillate by touching the input wit a pincet
The RF detector at the output goes high.

Does it stil have low noise i do not know. Soldering de-soldering needs high soldering power due to the big grounding pad
There is no soldering instruction in the specs but it must be the same rules  as for most chips

Next thing is making  a small adjustable negative bias supply and modifying the preamp with a -bias connection a bit

A  negative Voltage supply with the NE555  or 2 transistors 1kHz  AC oscillator both work okay.
But the whole thing became a bit more complicated.

Negative Voltage generating example here
https://www.allaboutcircuits.com/projects/build-your-own-negative-voltage-generator/

About 1 kHz AC generator with a 2 diode rectifier below will make nearly -5V  with  +5Vsupply
diodes IN60 r 100nF give about 1kHz the buzz. replaced with 500R
The collector output is connected to the negative rectifier circuit below.
Image result for transistor oscillator circuit

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<3v ...="" p="">So my SPF5189z   need  negative bias and i am the only Ham experiencing this?
Hard to believe  but no Google hits on the subject

                                                             40mA adjusted


                                                                  90mA  at - 0.64V bias



Comments

Milan Elezovic said…
I confirm that SPF5189Z from two EBAY stores works on 2.6V at 100mA.

Recommend power supply by Mini Buck Converter that does have smd chip marked AGCG.
It will step down from any 5V or higher source.
Jaap van oosten said…
Milan thanks for your reaction
It confirms my suspicion of this devices from China

Jaap van oosten said…
Milan thanks for your reaction
It confirms my suspicion of this devices from China

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