VLNA-144
144 MHz VERY LOW NOISE AMPLIFIER
by Gyula Nagy, HA8ET


Published in DUBUS 4/2019

 

Technical data:

Frequency range:…..……..144-148 MHz
Input/Output impedance: ... 50 Ω
Gain:………….………… 18 - 24 dB (adjustable)
Noise Figure………..……0,21 dB
1 dB Bandwidth:……….. 4 MHz
3dB Bandwidth:………..  5 MHz
Max input power………. 17 dBm
OIP3………….………..>20 dBm
Broadcast rejection:……..>63 dB @ 100 MHz
Rejection @ 432 MHz:…>60 dB
Unconditionally stable
Moderate sensitivity to ESD
Supply voltage:……...…  +12…15V/81mA
Powered through antenna feed line, or separately
+33 dBm IP3 GaAs FET (SAV-541+)
Connectors:…………...... BNC or N
Dimensions (Box):…….. ..74 x 37 x 30 mm

 

In practice, EXTRA-2 144MHz [1, 2] preamplifier has well lived up to expectations. However, the demand for a lower noise preamp has emerged from EME operators, so I developed a new preamplifier called VLNA-144.

Simulations

MINI CIRCUITS has published the s2p parameters of the low noise FETs on its website. Fortunately Modelithics Inc. measured not only the S parameters, but also the noise parameters. Big thanks for it! I tested the FMIN value of TAV-581, TAV-541 and SAV-541+ of the possible low-noise FETs with the help of ANSOFT SERENADE programme.  Fig. 1. shows that the noise figures of the 3 FETs hardly differ from each other. I chose type SAV-541+ [3], as it can be soldered with normal small-size soldering iron. This E-PHEMT has been developed to replace AVAGO’s (BROADCOM Inc.) ATF-54143 FET. S parameters of the 3 FETs can be seen in Fig. 2, while Fig. 3. shows K stability factor. It can be clearly seen that the characteristics almost coincide. Stability factor at lower frequencies is not sufficient, so feedback must be used.

Fig. 1: Noise figures of TAV-581, TAV-541 and SAV-541

Fig 2: S parameters of the FETs  

 

Fig. 3: K stability factors of the FETs (simulation) 

Fig 4:  Simplified input circuit diagram in simulator

 

For low noise and good dynamics I chose 4 V 60 mA setting. Fig. 4. shows the simplified circuit diagram of the input circuit. The 82 nH inductance with C1 and C2 capacitors forms an HP filter. After optimization, the HP filter provides a medium input selectivity with minimal noise. C1 and C2 are U series 0805 sized RF capacitors made by AVX. Their ESR values can be seen on Fig. 5. With QC=500 and QL=150, after the simulation and optimization, a noise figure of 0,11 dB was obtained (Fig. 6.). The S parameters of the simplified input circuit are shown in Fig. 7.

 

Fig. 5: Typical ESR vs. frequency 0805 AVX „U” series capacitors

Fig 6:  Noise figure of the simplified input circuit (simulation)

 

Fig. 7:  S parameters of the simplified input circuit (simulation)

Fig 8:  FR4 PCB features in simulator

For easy reproducibility, the input inductance was first developed as a spiral inductance on the PCB. Fig 8. shows the characteristics of FR4 type PCB in the simulator, while the parameters of spiral inductance can be observed on Fig 9. Fig. 10. shows the input circuit on the screen of the simulator.  I examined the characteristics of C1 and C2 with both ESR and Q values. According to the simulator, the noise factor remained excellent, but the practical measurements proved that I had to return to the original inductor, as in reality the noise factor increased (due to the low quality FR4 PCB).

Fig. 9:  Input spiral inductor features is simulator

Fig 10:  Practical input circuit in simulator

 

The noise figure of the final input circuit hardly increased compared to the theoretical value, it is only 0.14 dB (Fig. 11.) Fig. 12. shows the S parameters. It can be observed that the input circuit has some suppression of FM broadcasting interference. The stability factor of Fig. 13. remains above K>1 throughout the band. This is due to source serial feedback.

 

Fig. 11:  Noise figure of the practical input circuit (simulation)

Fig 12:  S parameters of the practical input circuit (simulation)

 

 

Fig. 13:  K stability factor of the practical input circuit (simulation)

Fig 14:  VLNA-144 circuit diagram in simulator

 

The full circuit diagram of the VLNA-144 preamplifier in the high-frequency simulator is shown in Fig. 14. This circuit has developed after many optimizations. To further improve the stability factor, an attenuator is applied to the output. The high side slope of the output band filter ensures the excellent selectivity.

The noise factor of VLNA-144 preamplifier is 0.14 dB, just like at the input simulation. (Fig 15.) If the attenuation of the output attenuator is increased, the noise factor increases as the gain decreases. Due to optimal noise matching NF approaches FMIN value specified for the FET. S parameters of the preamplifier can be seen in Fig. 16. S21 and S22 are excellent, but S11 is weak due to the effort to minimize noise. Markers are positioned at 145 MHz in the figure. The stability factor shown in Fig. 17. is excellent due to the feedback and the output attenuator. The preamp is unconditionally stable with any input or output impedance load

 

Fig. 15: Noise figure of the VLNA-144 (simulation)

Fig 16: VLNA-144 S parameters (simulation)

 

Fig. 17: VLNA-144 K stability factor (simulation)

Fig 19: PCB

 

FIG. 18: VLNA-144 SCH in PDF

Part list (DOC)

Construction 

The detailed circuit diagram of the VLNA-144 preamplifier is shown in Fig. 18. In addition to the high-frequency circuit, the circuitry includes a voltage regulator IC with appropriate filtering and a diode circuit (D6, L6) to make the preamplifier suitable to also be powered throughout its output.

The PCB drawing of the preamplifier and the positioning of the parts can be seen in Fig 19.

 Adjustments  

Adjust the current limit of a 13.5 V power supply to 100 mA and connect the preamplifier to it. The original factory settings of the potentiometer P1 (BIAS) require very little change to achieve the necessary ID = 60 mA value. In this case 78 mV must be measured between TP1 and TP2 points (IBIAS + ID). The total current consumption of the preamplifier should be approximately 81 mA. The power supply can be varied between +10 and 15 V, not influencing the operation.

After setting the DC conditions, potentiometer P2 (GAIN) must be set to maximum gain (turn fully clockwise). Tune the BP filter L3-L4-L5, using a spectrum analyzer and a tracking generator, until the maximum of the selectivity curve is at 145 MHz. This requires 1-2 turns inward from the original factory settings of both cores.

 

Measurements (TNX HG5AZB!)

 The 3 dB bandwidth is shown in Fig. 20.  The figure shows the excellent selectivity and suppression of broadcast frequencies. This is provided not only by the output BP filter, but also the input matching circuit contributes to it. Fig. 21. shows the transfer function of the VLNA-144 preamplifier between 1 and 1.000 MHz.

The measurement of the noise figure is shown in Fig. 22. FMIN=0.21 dB. The 1 dB compression point curves can be seen in Fig. 23. The Third Order Intercept measurement of the preamp can be seen in Fig. 24. (at maximum gain).

Fig. 20: 3 dB bandwidth measurement

Fig 21: Wide band transmission measurement

 

Fig. 22:  Noise figure measurement

Fig 23: 1 dB compression point measurement

 

FIG. 24: The Third Order Intercept measurement

 

Table 1.
 

 

Simulated

Measured

Noise Figure [dB]

0,14

0,21

 

Results

 Fig. 25. shows the photo of the ready VLNA-144 preamplifier. The bottom side of the VLNA-144 preamplifier is shown in Fig. 26.  Ready VLNA-144 preamp with different connectors can be seen in Fig. 27.

 I have tested approximately 10 pieces of the VLNA-144 preamplifier and the results are nearly the same. The circuit is stable and easily reproducible. The experiences have proved the measurement results and the simulation. It can be stated that the high-level behaviour and the selectivity of the preamp is outstanding in real-life situations, and the noise figure is certainly low enough. It was these positive experiences with the 144 MHz version that encouraged me also to develop the 432 MHz versions of the preamp. Fig. 28. shows the VLNA-432 MHz Very Low Noise Preamplifier. The Noise figure on the VLNA-432 is 0.29 dB.

Many HAMs like to use the pre-tuned (“boxless”) PCB version of the preamp (Fig. 29.) for modernizing their own transverters or outdated preamplifiers.

 

Fig. 25: VLNA-144 Photo

Fig 26: Bottom side of the VLNA-144

 

Fig. 27: Ready VLNA-144 Preamp with different connectors

Fig 28: VLNA-432 (NF=0.29 dB)

 

 

 

 

 FIG. 29: Pre-assembled and pre-tuned PCB

I would like to say many thanks again to the HAMs for the numerous supporting E-mails.

 

Literature 

1. Dipl. Ing. Gyula Nagy, HA8ET: EXTRA-2 144 MHz Contest Preamplifier. DUBUS 4/2010 p.78

2. Dipl. Ing. Gyula Nagy, HA8ET: EXTRA-2 144 MHz Contest Preamplifier (REV6). Dubus 4/2011 p. 70

3. https://www.minicircuits.com/WebStore/dashboard.html?model=SAV-541%2B

 

The VLNA-144 preamplifiers available:

-N male connectors
-N male N female connectors
-BNC connectors

Contact me..\Contact.htm

© HA8ET 2019