To evaluate the LNA/switch performance, a prototype was assembled on a printed-circuit board (PCB) previously designed for a non-bypassed LNA application.¹³ The PCB consists of 10-ml-thick RO4350B laminate material from Rogers Corporation, with a dielectric constant of 3.48 in the z-direction at 10 GHz.¹⁴ The PIN diodes and their associated biasing components were attached to the PCB by directly soldering them to the leads/pads of the earlier components. Two 1N5719 axiallead glass diodes15 were used as the switching elements for D1. In a later PCB layout iteration, these diodes will be replaced by an SOT-packaged PIN diode pair (model HSMP-3893/E).¹⁶

The LNA’s median gain is 19.8 dB with a 1.3 dB variation within the frequency range of interest [Fig. 6(a)]. The frequency response is flattened by gently attenuating signals below 200 MHz by merit of the highpass response from small-valued DC-blocking capacitor C2. The gain rolloff at the upper frequency end is consistent with the MMIC’s characteristics and possibly due to negative feedback through the unbiased PIN diodes’ parasitic capacitance.

In bypass mode, the circuit exhibits 3.8 to 4.5 dB attenuation over the frequency range [Fig. 6(a)]. Loss in this mode is mainly caused by the parasitic series inductance of the PIN diodes. To a smaller extent, the loss in bypass mode is also affected by PCB dissipation, the FET terminal impedances of the FETs, and the parasitic parallel capacitance of resistor R4. Still, bypass mode losses were well within the customer’s -5-dB specification limit, so now attempts were made to reduce bypass losses further.

Both input and output return loss were consistently good (better than 17 dB) in bypass mode when evaluated across the specified frequency range. Return loss is primarily affected by how closely the unbiased FET gate and drain approximate open circuits. The return-loss performance is not as good during LNA operation with worst-case performance of 7 dB at the lowest frequencies for output return loss. Poor output return loss below 70 MHz is caused by the small-valued capacitance C2 and is a tradeoff for a flatter frequency response.

Figure 7(a) compares LNA noise figure with and without ferrite bead inductor L1. The target noise figure specification (1.3 dB or less) cannot be met without L1. By comparing the traces, it can be surmised that signal loss attributed to the parasitic capacitance of resistor R3 is in the range of 0.3 to 0.6 dB, increasing the noise figure by the same amount. With L1, there is more in-band variation in noise figure (0.5 dB versus 0.2 dB), but this is not critical. The increased variation is likely caused by the ferrite bead’s progressively reduced choking capability with increasing frequency, especially above the 100 MHz or so self-resonant frequency (SRF) estimated from the manufacturer-provided performance graphs.¹⁷

The LNA’s output third-order intercept point was measured as several evenly spaced frequency points within the mobile television frequency band using a two-tone input power level of -20 dBm. The input third-order intercept point was calculated by subtracting measured gain from the outputthird- order-intercept data. The output third-order intercept point doesn’t go below +30.3 dBm with maximum variation of 0.8 dB within the band [Fig. 7(b)]. The linearity improvement of about 10 dB over the data sheet’s nominal value (+20 dBm)¹⁰ can be attributed to this design’s higher I_ds (30 mA versus 10 mA).

The LNA/switch design satisfied its target specifications and shows great promise for improvement. For example, the noise figure can be improved by replacing the current ferrite bead inductor with higher SRF versions.

ACKNOWLEDGMENTS
The author would like to thank M. Sharifah for assembling the prototypes, Ray Waugh for the mentoring on PIN diodes, S. A. Asrul for reviewing the paper, and the management of Avago Technologies for approving the publication of this work.

REFERENCES

1. “Mobile TV,” Internet: en.wikipedia.org/wiki/Mobile_TV.

2. “In car entertainment,” Internet: en.wikipedia.org/wiki/In_car_entertainment.

3. C. Baringer and C. Hull, “Amplifiers for Wireless Communications,” in RF and Microwave Circuit Design for Wireless Communications, L. E. Larson, Editor, Artech House, Norwood, MA, 1997, pp. 369.

4. Philips product specification, “FM1236 Desktop video and FM radio module,” February 1997.

5. L. Federspiel, “Clear RF reception on mobile TV Technology,” EE Times-India, February 2008, pp. 1.

6. D. M. Duncan, “AGC in Transistor Broadcast Receivers,” IRE Transactions on Broadcast and Television Receivers, July 1962, pp. 125-134.

7. Ulrich L. Rohde and T. T. N. Bucher, “Amplifiers and Gain Control,” in Communication Receivers, International edition, McGraw-Hill, Singapore, 1994, pp. 235-237.

8. C. L. Lim, “Adjustable Gain Cascode Low Noise Amplifier,” High Frequency Electronics, July 2009, Internet: www.highfrequencyelectronics.com/Archives/Jul09/0709_Lim.pdf.

9. W. Hayward and D. DeMaw, “Receiver Design Basics” in Solid State Designs for the Radio Amateur, ARRL, Newington, CT, 1986, pp. 88-89.

10. Avago Technologies product specification, “MGA-68563 Current-Adjustable, Low Noise Amplifier,” Internet: www.avagotech.com.

11. Vishay Intertechnology, Inc., Application Note AP0010, “Resistors in Microwave Applications,” Internet: www.vishay.com.

12. Avago Technologies application note 1049, “A Low Distortion PIN Diode Switch Using Surface Mount Devices,” Internet: www.avagotech.com.

13. Avago Technologies application note 5011, “MGA-62563 High Performance GaAs MMIC Amplifier,” Internet: www.avagotech.com.

14. Rogers Corporation product specification, “RO4000 Series High Frequency Circuit Materials,” Internet: www.rogerscorp.com.

15. Avago Technologies product specification, “1N5719, 1N5767, 5082-3001, 5082-3039, 5082-3077, 5082-3080/81, 5082-3188, 5082-3379 PIN Diodes for RF Switching and Attenuating,” Internet: www.avagotech.com.

16. Avago Technologies product specification, “HSMP- 389x Series Surface Mount RF PIN Switch Diodes,” Internet: www.avagotech.com.

17. Murata product specification, “BLM18R Series (0603 Size),” Internet: www.murata.com.